I rely on PC and what it has but I do use specific third party plugins for things. I have a lot more than I really use.  

 
I use outboard hardware going in, like pre amps and compressors, but none going out.
Once it's recorded to Sonar it never leaves "the box".   
I might use an outboard Midi sound module but that ends up ITB as an audio track.  
Sonar is my recorder, editor and mix down deck. I use only Sonar plug ins and Sonar synths. Mr Tramp is my only 3rd party synth. 
I then master in Wave Lab using all Steinberg plug ins. 
CD's are burned using Nero.
MP3 are encoded using Gold Wave.   

I certainly could mix with only what comes with Splat (or Producer before that), especially with the PC and extensive range of other plugins included. I went through a bit of a plugin explosion a few years ago, then realised I actually wasn't using most of them that much, I was better off learning a few than playing around with lots and I was getting swamped by choice. So I weeded out most of them a few years ago.

Apart from what came with Platinum I currently have a few Waves plugs for specific jobs (Reel ADT, Abbey Rd tape, two or three exciters and the one-knob pumper which is dead simple and surprisingly useful). Also the VKFX delay and chorus and BOZ compressor/eq, all of which have something quite distinctive about them. Also a few amp/pedal sims which despite repeated tries to get along with I don't use much.

If I need a vocoder I've a pretty good one in my Waldorf microQ, but I agree with Craig that Sonar really could do with its own vocoder. Also a good panner, though I'm using the Melda free one at the moment, which does the job very well so I can understand Cake not wanting to put much effort into developing their own. A good step/lfo programmable tremolo and on-the-fly vibrato would be nice as well.

Other then the vocoder, the third-party stuff is really for specific colour rather than because I'd be lost without it though. I could mix using Sonar alone and I have done.

dubdisciple
Your Vintage Echo  is my goto delay for "mixtape" like "hype" tracks for a few rappers i mix for.  I agree on the maximizer. 

Glad to hear that! Having used real tape for so many years for echo, that sound is burned into my brain and I wanted something that did it.

I've been mixing with Sonar for more than a decade. I didn't start with the pro version. I slowly built up. Consequently, I've used all the Sonar plugs a lot over the years. I slowly have bought more and more stuff. Pro Channel, Vsts, drum kits.
 
I have a package of Nomad, Overload and Melda Productions plugs.
 
Honestly, EQ is the most important plug by a long shot. Compression falls second and works in conjunction with EQ. Lots of the plugs I like basically help me use EQ and compression together in creative ways.
 
I think if I could go back 20 years and give me some advice it would be to twiddle those EQ nobs while listening closely at every opportunity.

Outside of the various multi-faceted plugs Sonar provides - I only find somewhat of a need to go outside when finalizing/mastering the tracks.  I like Voxengo Elephant and some of Voxengo other plugs in the MASTER BUS :-).  Is it necessary and/or does Sonar have something comparable or better (LP-64) - maybe - BUT I really like Voxengo in that final bus IMHO...

Mixing completely in Sonar is getting easier all the time. I do still use my Waves CLA Compressors but, not as much as I used to. I love Sonar's CA2A and the Boz +10db comp (I consider it a part of Sonar because it's a PC module). I've never been fond of the other PC comps.
The Quad Curve EQ is great (Kinda wish it had a couple more bands, though), as is the Boz EQ. I've messed around a bit with the Nomad stuff that came with Sonar and am pretty impressed with what I've found.
Melodyne works super with Sonar.
As far as a final, mix bus limiter, I've been having great lick with the Concrete Limiter. I know, everybody says it's not up to the task but.... I don't know.... I'm pretty impressed with it. I used to use the Sonnox limiter or a Waves L series limiter but, I really prefer the Concrete Limiter.

Synths aside, other than always using Ozone for "mastering" I use the plugins included with SONAR I'd say 80-90% of the time.  That 10-20% isn't consistent by any means, but aside from specialty items like Melda's DynamicEQ or amp sims it's most often some very specific flavor or effect.   And for the latter I'm testing more FX Chains for that purpose, mostly since Craig's Platinum contributions have renewed my interest in them; lovin' those new amp sims. 
 

All you guys saying that Sonar needs a vocoder....  Can it be assumed that the one in Pentagon has been tried?

Pentagon is no longer bundled with SONAR
To the topic, I still use SoundForge and few third party plug-ins more out of habit than need.