Title says it all. I used a slow leveling mod in FO3 because my game style is to visit every pixel of the game board and do every quest, so I enjoy slowing down the leveling as it adds more enjoyment at the end.

http://www.newvegasnexus.com/downloads/file.php?id=35178

This feature and many more are part of Arwen's Realism Tweaks:
http://www.gamesas.com/index.php?/topic/1133258-relz-arwens-nv-realism-tweaks-thread-2/

A vote for http://www.newvegasnexus.com/downloads/file.php?id=35015 here.