Flaming Pear  
Free demos available
Super Blade Pro
Adds textures and bevels and reflections for a 3D type look
Super Blade Pro and Blade Pro are extremely popular. I like all of Flaming Pear's products.
Hue and Cry
Creates interesting patterns.
Lunar Cell
Design your own planets, lots of controls to work with
Flood
Realistic pseudo-3D watery reflections
A" must-have" filter
Designer Sextet
Six filters, includes Aetherize, Twist, Swerve, Lacquer, Silver, and Glare. Aetherize is one of my favorites.
Four Filter Special
Four filters,. includes India Ink, SolarCell, Tesselation, and FeatherGIF. 
SolarCell does some nice starbursts and suns.
Knot
Filter for twisted 3-D abstractions

Also these free filters
Vitriol, Color Swap, ChromaSolarize, 
Tachyon, Ghost, Make Opaque,  Primus

Font View  
Free program
Program for viewing and testing your installed fonts. You can
experiment with the size, color and style of a phrase to help you choose
which font to use.

Eliminate Black Filter  
Free filter from Joe Cilinceon. it does just what the name says

Fantastic Machine  
Fantastic Machine's Tile Tools, Filters  for creating tileable images and textures
Fantastic Machine's Paint Engine, Free filters for painting effects.

Filters By Sandy Blair  
Useful and fun filters
Simple Filters      Nue Filters     

Fire, Flutter, Lead Emboss  
All three of these sets are free filters

Filter Factory Galleries  
Large collection of free filters

Filters Unlimited  
This is one of my favorites. It's a program for organizing, using and creating filters. 

Free Filters @edesign.com  
Free
Eliminate White Filter

Fractal Explorer  
Free Program
Generates fractal images and animations

Filter Farm   
It's not a filter. It's an environment where filters live their own life.