I had a ton of plugins before, but now, for a year, i don't use any plugins anymore, just genuine PS filters that come with the installation.
But plugins are useful for starters to make their creativity better
This is the list of plugins i recommend (i'm not sure if they all are shareware):
1. all KPT (Kai's Power Tools) plugins, i had 2,3,5,6,7 versions
2. Eyecandy
3. Xenofex
4. Alien Skin effects
5. Photographic Edges
these kinda the most expensive ones and my best ones as i remember
http://davinci.ice.org/index-0.html - take a look at the work of this PS guru
cheers