Many, many mods are available. In the thousands. The best collection of mods I've seen so far are on www.fallout3nexus.com. Every mod is categorised and described in detail, just do some browsing and you'll find what you need.
At the fallout 3 website (www.bethsoft.com) download the geck to make mods!
Yes, you can install Fallout 3 PC mods from Fallout Nexus onto the PS3. For a tutorial, see: http://fallout-mods.blogspot.com/2010/11/fallout-3-mods-for-pc-on-ps3.html
there are no PC mods for Fallout 3 on PS3. Why do think they call it Personal Computer
just mods basically
fallout3nexus.com
Yes.
You don't. It does not exist in fallout 3 without the use of the console and mods. ( Only for P.C )
In Fallout 3, there was a site called "Fallout 3 Nexus", from what I understand it was where everyone went for mods. Similarly, one was made for New Vegas named "Fallout New Vegas Nexus". You'll be able to find all the mods you could ever hope for there (just google it). Be sure to download the Mod Manager, it makes mods easier to use.
Nope. Fallout 3 and Fallout: New Vegas are completely different games, sure they are in the same title and the same game engine, but the configurations and in-outs are different for both games, so you cannot simply apply a mod you used for Fallout 3 and try to run it on Fallout: New Vegas. You may be able to find mods that match your Fallout 3 mods for New Vegas on an assortment of different websites all over the net.
You can't use mods on Mac or iMac, it only works on PC.
Update your mods, update your Fallout 3 application with the latest patch, and make sure you installed your mods correctly. An exclamation mark usually means you have a missing texture or that there is an error in the bsa file.
Joining enables you to download bigger mods so if you like mods, Yes.