Nowadays I mostly use SFX! and its variants (a system of my own devising), though I do use Stars Without Number for a science fiction game with the kids, and a homebrewed version of D&D when they want to go dungeon delving. I also recently ran a one-shot of Old School Hack.
As a player I play in my friend Mac’s homebrewed D&D game, and in Russell’s homebrew he calls brew 20, as well as the SFX!-based campaigns my friends are running: Russell’s Surf City campaign using Argh!, and Dan’s Warhammer 40K campaign that recently converted to Zap!
In the past we did a bunch of Savage Worlds, D&D third edition, FUDGE, and others too numerous to mention.
Tales of the Rambling Bumblers 
Columns on what criteria you use to decide whether you want to adopt a game system and on how you go about designing your own systems would be interesting to me,