There is a useful one called the Darn UI mod, which makes all the big fonts smaller and lets you see more entries at once in various on-screen lists.
The weight limit was annoying at first, but I have gotten used to it now. The key is to only pick up things with a good weight/value ratio, like at least 1:10 or 1:15. Maybe I'll try out that mod for it though.
Is anyone using the improved texture mod? I haven't tried it out yet. The best thing would be if the guy who made the Oblivion texture pack created a similar one for this game (the existing one is made by someone else). The textures in that mod were amazing.
Improving run speed is a good one too. It wont unbalance the combat but it will speed up everything else. I think you can adjust that with a console code though.
I am playing the game with the iFPS microstutter fix (IMO the game is simply intolerable without it), which has the side effect of making everything go at double speed when the framerate is over 60. This is actually a good thing most of the time though, as it means you waste a lot less time running around between places. It does mess up the lipsyncing and alt-tabbing, but it's still better than the alternative.