There is also Rapture Pro, which is like a combination of Dimension Pro and Rapture, and there are some REALLY good pads, as well as a good set of both sampled sounds and synth sounds individually.  That is a cost effective option.  It comes with the Dimension Pro sound libraries, as well as Rapture sounds, and combination layered sounds.
 
For FREE, you could try out the Native Instruments Kontakt Player and the Reaktor Player.  Both come with a usable set of sounds.
 
My 3 sets of Go To sounds come from:
 
NI Komplete 8 Ultimate
Arturia V Collection
EastWest Composer Cloud
 
(I also frequently use some of the Cakewalk sounds)
 
Bob Bone
 
 
 

I thought all we had to do was take our finished track and run it through LandR, right? ...poof..instant professional sound!
 
That was a joke of course. The tutorial videos from Groove3.com helped improve my mixing/psudo-mastering skills dramatically (they are cheap, but not free).  Youtube is a great place to learn from as well.
 
However... the #1 tip I can think to provide here is to listen to your tracks up against other professional songs in the same genre (even if it is only on your headphones. *Magic AB is a fantastic plugin for this). If the pro song has more high end, try adding some high end to your tune. Does your tune sound boxy compared to the pro tune? Scoop around in the lower mids. Are the vocals louder in the pro tune? Turn up your songs vocal. etc...etc... 
 
You will have to decide if adding more highs to your song means turning up something like the cymbals, or do the cymbals themselves need more of a high end EQ adjustment. Go back and compare it to the professional song and figure it out. Make an adjustment on your mix and then go back and compare.
 
I'm not saying to try to exactly match a tune's tone, but to select a few songs that you like. Something like one song that you like the low end punch on, another professional track that you think has a nice sheen. Another where vocals shine through.
 
Just make sure you turn the volume down on these reference songs to match the volume of your own tune. Once you are happy with your results, a mastering engineer can later make your song as loud as the professional ones (in theory) -- or you could then use LandR for final mastering -- Or you could just smash it through a brick wall limiter to bring the overall volume up. (That last part is a joke too...)
 
I think everyone here is chasing the perfect mix for our songs. You are not alone in the search.

One more thought about fat synths...the Z3ta sounds are pretty incredible ..just gotta sift through to find what you can use

Just one other thought.  Maybe from a little different perspective. Songs themselves have to be great. The performers have to be great in order to create a great song.   I don't think I have ever heard of a song being great because is was recorded with pristine audio.  Some of the great classics really don't have great sonic qualities, in fact,  you can often hear pitch issues, mistakes etc. in some of the classic rock songs.  I think we should always strive for great audio quality, but not be confused about its value in creating great music.  Elvis often recorded his songs live to 2 track with gear significantly below the quality of what we are using.   Finally, as a front of house guy, don't underestimate the value of mixing.  Knowing how to EQ a mix is critical, especially subtractive EQ rather than additive.  Some inexperienced folks try to maximize the quality of each instrument, and it sounds terrific when soloed, but when put into the mix they compete and sound awful.  
 
As others have suggested, practice, know your gear, understand the music and remember what I learned a long time ago, most of the time it's not the gear, it's the musician.  When I played in a band as a teen, I thought our problem was mostly our equipment until a really good group of musicians sat in, using our equipment and absolutely blew us away.  We were astounded, but learned an important lesson that day.  Learn your craft.  

Heh, as someone who makes music purely as a hobby, I often think about the following reality:
 
"Professional" music has professional musicians writing the music, with professionally trained musicians each playing an instrument they specialized in, with dedicated professional engineers each doing the recording, mixing and mastering, all on expensive professional gear which they know intimately.
 
On the other hand, I need to write my own music, do my own arrangement, "play" each instrument myself, do my own vocals (nooo!!!), do my own recording, mixing and mastering on gear that might actually be good but of which I don't really know much about.
 
So the chances of something "professional" sounding coming out on the other end for me is practically zero

I assume you are mixing in a small space.
You need to
1) invest in some good studio monitors. the $500 to $1000 kind (not the $10K+ kind). Headphones will not cut it.
2) acoustically treat your mixing area at least to get rid of early reflections and also possibly some bass traps. Plenty of advice about this on the web.
Use reference mixes of material you think sounds good, get accustomed to what those sound like on your rig. Listen to your mixes on different rigs.I use a typical stereo , a boom box,  the car stereo and a couple of rigs my friends own. Try your mixes in mono...they should still sound good.
Once this is done many, many times , you will begin to 'learn' your space and your monitors and what your mixes will sound like on various playback devices.
I wish there was an easier way but unfortunately there is not.

SoundRegion
I'm using Sonar Artist and I'm trying to get a decent end result with my music. When I mix the music in headphones it sounds OK, but when I play the music on the big stereo it does not sound very proffesional. Muttled audio, samples, especially drums sounds bad. Even synths. I'm using primarily samples that comes with the product, like SI instruments and I can accept that there might be better alternatives out there, but I have tried a lot of different synths and just can't understand why synth music from the 80's still sound better better than mine. Also do my type of sound card have any influence on the finished product other than maybe S/n ? I use an Akai EIE pro. I was thinking about the Native Komplete 10 bundle. Will that make a big difference in my sound over the standard Cakewalk sounds ? I have tried a lot of the synths and not that impressed, but maybe the samples are worth getting ?

Imo it´s very demanding to create good sound and mixes.
That alone is an art which needs years of experience and diligance.
To do that by headphones is even more difficult, specially if there is not much
expierence in creating good mixes aso with monitors.
 
I had hundreds of mixes which sounded great via headphones 
and collapsed  by hearing via studio monitors.

There are really only two options. Become a good mix engineer, or hire a mix engineer. The former takes years, and costs money, the latter just costs money.
 
I will say this: monitors and room. There are no hacks and no tricks. You need good enough monitors in a good enough room. Doesn't need to be Abbey Road or anything, but all your other efforts are irrelevant without good enough monitors in a good enough room.
 
And while there are many here who will protest, I don't hesitate to say this: stop mixing on headphones. The list of people who can make translatable mixes on headphones is incredibly short, and entirely made up of absolutely top professionals with many years of experience. It is certainly the absolutely worst option for when you're still learning.

Sorry, I couldn't get passed "proffesional".... an amusing irony.
Fortunately, others gave some very PROFESSIONAL advice and tips.
 
 
 
------- Lee Shapiro
          Musician, Songwriter, Spelling Enforcement Officer

Probably your best, ie. cheapest means to get a great collections of sounds is to upgrade SONAR.  SPAT has great drums, effects and Dim Pro and Rapture, among other "Professional" synths.  Cake does deals on and off.  That is a great and reasonable way to upgrade your sound.  NI also does good deals - mid-summer and Christmas.  If you can wait you can find a lot of their stuff at half price.  Both are good choices.