It depends on whether I'm the DM or not. If not, I don't ban anything.
If I am the DM, it depends on the campaign. Typically...I only allow what's found in
The core rulebooks
The Complete books
PHB 2
Dungeonscape (probably because I have a factotum fetish, he's the real jack of all trades).
Except for that, if it's a Forgotten Realms 3.X game, anything in those are allowed. If we are going 3e instead of 3.5, the equivalent of the complete books (like Sword and Fist) are allowed.
Typically, outside of FR, the following modifications are in the game...some for simplicity, some to keep me sane on book keeping.
We use Unearthed Arcana options.
All skills are chosen at first level...with alternates if you ever multiclass. We play with the max skill points option from UA
You are only ever allowed to multi into 3 classes period...and they all have to be different areas (like Warrior, Wizard, Cleric, or Warrior, Rogue, Wizard...no Fighter/Rangers).
Any Prestige classes must be planned from the start (goes along with the max skill ranks idea). You are an apprentice or initiate into that organization. Whether you suceed and become one of them is dependant on you...and how successful you are in the future (aka...if you meet the prereqs you get the prestige class when ever you want to start taking levels).
If you want to suddenly change direction and multiclass into something unplanned, you can do so ONCE in a campaign...IF YOU HAVE A GOOD ROLEPLAYING reason to do so...(aka...no...I just felt like it to pick up such and such a skill). Note: If you plan from the beginning that you are a Fighter/Duskblade or whatever...no such penalty/reasoning exists. It's part of your gameplan from the beginning.
I have that races have a different psychology than humans. Elves are NOT simply humans with pointed ears. This psychology and different outlook is why we use the options from the DMG restricting certain races from playing certain classes. In addition, you better play as if you are a different race. NO...I'm an elf but I act like a human. Act like the elf you are.
This keeps the levels and classes at a level where I can usually know everything that they are able to do and track it all. Everything else I can counter as a DM.
Be a munchkin as a Druid...WOW...where did this monster that has a 10,000 DR/Magic (or whatever I make up) with it, which in this case means ANY damage caused by magic or RESULTS of magic, inclusive of buffs that add their damage, attack bonuses...etc. to it. In addition it reflects all that damage back on the person using the shapechange, buff, spell...etc. Wow...what a terrible creature...I'd run away from it...but that's just my advice...do as you will. Basically, if they go crazy...I can normally use anything in my imagination with rule 0 to counter it and balance out the party, so all players can feel like they are a benefit to the party if they wish to participate (so no one sided Wizards, Clerics, or Druids rule the day...it is a party thing where all work together to build on their weaknesses and strengths).
Other than that, feats, weapons, spells, whatever they want from the books above...they are all allowed.
Occasionally I'll run a Corebooks only campaign...it should be obvious what is or isn't allowed in that.
I also occasionally run a stronger UA campaign, where we use the generic classes from UA (Warrior, Spellcaster, Expert) along with choices for the prestige classes (Bard, Ranger and paladin).
Occasionally I also allow Gestalt characters to be run alongside with normal characters...but they take 2x the XP to advance to the next level.
So I run the gamut of totally open in Forgotten Realms...to more closed games.