Everything's case-by-case with me, but 90% of what my players use comes from the three (not four) core rulebooks. Players have the option of using the Psionics Handbook classes as well, but to date no-one has done so although one player was seriously considering it at one stage.
Some examples of non-core things that have appeared:
* Numerous monsters, some heavily modified by me before actually seeing play. Just because my players largely ignore non-core books doesn't mean I have to!
* A selection of about 120 feats from various sources, mostly WotC splatbooks. I give my players a printout of this, so it's prety much the only non-core resource they actually USE.
* Some variant races players have asked for, if they fit the world.
* The variant Bard and various spells from the Book of Eldritch Might series
* Two homebrew prestige classes
Overall it's a pretty core-rules based game even though I'm quite open to it being otherwise.
Some examples of non-core things that have appeared:
* Numerous monsters, some heavily modified by me before actually seeing play. Just because my players largely ignore non-core books doesn't mean I have to!
* A selection of about 120 feats from various sources, mostly WotC splatbooks. I give my players a printout of this, so it's prety much the only non-core resource they actually USE.
* Some variant races players have asked for, if they fit the world.
* The variant Bard and various spells from the Book of Eldritch Might series
* Two homebrew prestige classes
Overall it's a pretty core-rules based game even though I'm quite open to it being otherwise.