Filter Meister: Tool i used to realize my plugins. It's addictive and very powerful, unlike the older Filter Factory and Filter Formula. Its language is inspired of C.
The Plugin Site: Offers valuable plugins (Some are free). Interesting resources to help you to choose or to use plugins. Also a great newsletter.
Cybia: Offers many free plugins. Each one allows multiple variations with these effects: transparency, color, edge, emboss and grid.
Plugins World: Huge directory for Adobe & Apple plugins. Quickly find what you need.
F.lux: (free) If you spend too much time in front of a computer monitor, I know two ways to reduce eyestrain. The first one is to have eyeglasses with tinted lenses. The second one is to just use f.lux. Actually, I do both.
Meazure:(free) A too useful tool! Allows to measure distances and angles on the screen.
Morguefile: free high resolution digital stock photography for either corporate or public use. Use the search engine and find almost anything that could suit your creative projects.
Imageafter: through a large photo collection, you can perform an "abstract search" i.e find photos by brightness, contrast, crispness...