A great place to find more mods than you can handle is at the Nexus website. Create an account there and you can access all the best mods available. The site also has a section dedicated to Oblivion made by the same game developers.
At the fallout 3 website (www.bethsoft.com) download the geck to make mods!
Joining enables you to download bigger mods so if you like mods, Yes.
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.
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 )
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.
Sadly, since there is no File Sharing system in place for the Fallout 3 game, you cannot download them on the XBox 360. But if you have Xplorer and Xasta, then you can download XBox 360 oriented (Meant for the XBox 360 and not pc) and port them into a saved game file.
You can't use mods on Mac or iMac, it only works on PC.