I'm with EOL, usually just change a few names and stuff. Sometimes i'll alter the monsters in modules to make them fit the campaign more. I run in Mystara so theres quite a few unique monsters.
For feats, classes, etc. i'm willing to run them as is. If they truly suck or are totally broken then i discuss it with the player and tweak it a little. I usually don't presume to know what is 'balanced' or not just by looking, especially since every group is different. I have a list of stuff i've tried already and decided to tweak, it goes into my campagn handout so everyone is on the same page. Mainly Bard skill points to 6+, Skill Focus, and 2 weapon fighting. And the If Thoughts Could Kill psi rules.
Companies have a tendency to get really game-breaking with spells, though, so i review non-WotC ones first. If any spell becomes a problem i look to see first if its my fault ommiting something, or indeed the spell needs altering. Most of the problems i've read on these and other boards stems not from broken rules, but bad DMing.
I'm a pretty liberal DM though from what i've read on some boards.