Fallout 3
-Wanderer's edition
rebalances the games, and you can tweak the game's mechanics. such as limb damage and how much skill points INT gives
http://fallout3.nexusmods.com/mods/2761
-the one unofficial patch
http://fallout3.nexusmods.com/mods/3808
-Fellout, makes the game less green
http://fallout3.nexusmods.com/mods/2672
-Texture pack
if you have a good computer
http://fallout3.nexusmods.com/mods/12056
-More Perks
http://fallout3.nexusmods.com/mods/6190
-Project reality, adds weather, and changes lighting
http://fallout3.nexusmods.com/mods/17418
Fallout New Vegas
-Project Nevada, rebalance mods
http://newvegas.nexusmods.com/mods/40040
-More Perks
http://newvegas.nexusmods.com/mods/35979
-Mod Config menu, config mods
http://newvegas.nexusmods.com/mods/42507
-Unified HUD project, lets you combine UIs
http://newvegas.nexusmods.com/mods/38961
-Warzones, add areas with lots of NPC battles
http://newvegas.nexusmods.com/mods/40980
-Nevada Skies, add weather
http://newvegas.nexusmods.com/mods/35998
-electro-city, adds light poles along roads
http://newvegas.nexusmods.com/mods/37908
-MTUI, makes the menus larger so less time scrolling
http://newvegas.nexusmods.com/mods/34902