I dont think we used any special add ons in the past as they are pretty expensive and we dont have any extra money. Some can cost upwards of $500. Most of them do one or two things, such as Shag/Hair creating Hair. Max Matter is also a cool addition to materials if you are planning on having a city in your animation(it has other uses) and Particle Studio is pretty cool I think. Other than that, I cant say Ive used many other plugins. Plugins can make a difference in your animation because of their specialization. If you are going to do a lot of particles, buy particle studio. Likewise, If you are going to do hair, buy shag/hair. But they are quite an investment so I would figure out exactly what you wanted to do and then go find the plugins you needed. Oh yeah, you should check out the free plug ins at
www.3dcafe.com, some of them arent bad.