Yeah, what Darius said.
I really can't stand the way plugins limiters 'chew up' the audio so badly.
The one plugin that I think might fit the bill is the PSP Vintage Warmer.
I got it several months ago, and I didn't spend enough time with it to get it sounding the way I wanted it to. Alphajerk was helpful; he took the time to explain some of the concepts to me. I've been using it more lately, but I find that to thicken things up without 'chewing up' the sound, it's best to leave the knee at zero and bring the drive control up until you get the desired sound. I only use it in multi-band mode, and I do get some use out of the release muiltipliers-per-band on the back of the plugin (although it's a drag to have to walk around to the back of the screen to adjust those knobs

) When I figured out to leave the knee at zero, though, it started to get a lot more use. Until I can dig up the cash for a good outboard mixbus comp.
I haven't really found any plugin EQ's that I think fatten things up the way I like. The Tritone is good, and the URS are good, but I only use them sparingly.
I also get a good deal of use out of the Ohm Force Frohmage. It's free, and it's a weird filterbank. But you can find a nice 'fattening' setting sometimes, if you keep it subtle. I use it on guitars and basses often. I set it up such that you can't tell it's there until you take it off, then you miss it.