Fallout New Vegas
Fallout: New Vegas Mod - A Requiem for the Capital Wasteland v.9b
This is a mod that allows you to play Fallout 3 within Fallout New Vegas. However, this does require that you have purchased both FO3 and FNV, or else the mod will not work.
Basically this adds a sewer grate that teleports you in between Vegas and DC. But the real marvel of the mod is that you can play Fallout 3 quests, as if it was the real game. Mods do work for both FO3 and FNV, so no problems there. The FO3 mainquest is a bit buggy but we're ironing those out.