I second the recommendation for Unearthed Arcana.
The character variants cover races, classes, class features and magic. They are all very good except the races (which are everything but original), but on the other hand the bloodlines in that chapter are very interesting.
There are 2 variants on characters "as a whole", that is they are supposed to be applied to every character and substitute the PHB class set: gestalt characters and generic classes.
Gestalts can be at best a way to try interesting multiclassing a'la AD&D 2ed, and at worst can be a tool for teen players who used to complain that "spellcasters suck because they are too weak", "warriors suck because they can't cast spells" and so on...
Generic classes are useful if you want a game with characters who are more feature-based rather than really class-based (they could have gone further and put a variant with a single class).
Then there are a lot of rule options about many different things: skills, equipment, combat, defense and so on. I am not very fond of these variants, since I like the core rules a lot and I think they are still the best option. But in general I hear that the majority of messageboarders cannot live without variants of house rules

so you may want to check out options from this book.