Veloute and ShaderOps all the time.
Anything grows frequently
Anything goos, just got it but looks useful.
Anything grooves, almost never (nice concept but there are often better ways to accomplish this affect)
Starbright, occasionally but wouldn't recommend it (could stand to be redone)
Shader Gel: use it occasionally but nice when you need it
ToonPro, just got but love it. (Here's something I did with ToonPro + lots of postwork).
http://www.renderosity.com/viewed.ez?galleryid=1050637&Start=1&Artist=marcq&ByArtist=Yes

ShaderPlus: I'd only get if you have a specific need for it but useful.
Seems like I'm forgetting a few but I don't use the others much.

For particles, I really like ParticleIllusion which is a 2D, post-render tool. I find it much better than the existing Carrara particle system. An enhanced particle system in Carrara would be nice but for most of what I do, PI is better and faster. But then again, I don't do animations. Most of the particle effects in my gallery are PI.

Marc