Not in any order:

Voxengo Span - Free Spectrum Analyser 
Ambience - Reverb
Kjaerhus Audio Classic Series - Effects Bundle
TAL - Togu Audio Line (various synth, effects, etc)

Don't forget that the bundled Sonitus effects are very good and quite underrated.  I really like the Gate as it can be side-chained and uses minimal processing power.  In fact, I think it's better than using a compressor to get that ducking effect as it doesn't colour the sound at all.

I also like the reverb as I get good results, very smooth and lush but one downside is that it only has a mono input so I only use it with mono sources.

Lastly, I have to say that the Delay is excellent too and I've had great results from that so some excellent plug-ins within Sonar.

free bundle and sound half decent:
my theory is you have to pay if you want to lay.
but theres some pretty good stuff out there, in a later project at some point id like to try and record a whole demo using just freeware.
http://sleepytimedsp.com/software/str-bundle

Sleepy-time dsp is some good stuff.

1) Alchemy Synth
2) Baxter EQ by VOS
3) Omnisone by Toneboosters

chuckebaby

but theres some pretty good stuff out there, in a later project at some point id like to try and record a whole demo using just freeware.

http://www.groove3.com/str/Mixing-with-FREE-Plug-Ins.html
 
Ironically, the video is NOT free. 
 
Cheers,
Eddie

 

This one I came across a few weeks back

Virtual Studio - Beyerdynamic
It works wonders as an FX bus with sends to that bus. Adds dimension to a mix 

Ok, this TDR Feedback compressor II by Tokyo Dawn Labs  that everyone keeps mentioning. WOW. I just tried this out. Got to be the most transparent compressor I've EVER heard in my life! This thing is amazing!! I just tried it out on drums and smashed it hard. Sure, it sounded compressed, but no matter what setting I chose for the release, it didn't pump! Every comp I've ever used to hit drums makes the cymbals swell if I hit it really hard (which sometimes I like the sound of if I'm going for that), but this thing just maintains perfect tact the entire way through! Zero pumping! Amazing. This would be so great for a mastering compressor.. So smooth.. Definitely adding this comp to my arsenal... It does NOT sound like a free comp in any way. Can't wait to try this on other things. I'd love to see how it compares to the PC2A on vocals.. Must. Go. Test.

I used to have a ton of freebies, but I got rid of almost all of them and my Sonar is much more stable without them. With Sonar's effects and the Pro Channel I don't think I need any others.

Free VSTs by manufactures of quality paid plugins are the best bet if you are worried about stability. For effects I like Blue Cat's free stuff.

Antress Modern plugins.
GVst series