http://www.fallout3nexus.com/downloads/file.php?id=3211 - This adds a lot of variety, and lets interiors respawn.
http://www.fallout3nexus.com/downloads/file.php?id=2761 - This completely overhauls the game, but comes highly recommended. It includes a global respawn timer option, which you can adjust, and also lets various containers respawn, too. With the FOIP patches, they work peachy together, too. ^_^
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Or, if you want to keep it simple...
http://www.fallout3nexus.com/downloads/file.php?id=1252
http://www.fallout3nexus.com/downloads/file.php?id=2519
http://www.fallout3nexus.com/downloads/file.php?id=515
:)