Unlike most of the mods I've been posting, EngBags is a pretty new one to me, but I'm already in love with it. It's an all-in-one bag window that sorts your stuff for you -- trade goods go in one area of the window, quest items another, etc. Brilliant and a little bit demented. It's highly configurable, which can make it seem a bit daunting, but the default config is pretty good. I always found keeping my inventory organized a bit of a chore, and now I've got a mod to do it for me (though I do occasionally have to reassign things' categories for best effect).
Advanced Bags Plus is a slightly different solution for the same problem. It replaces your five bag icons in the bag bar with a modifiable number of virtual bags for different item categories. It seems slightly easier to configure than EngBags in terms of which items should go in which categories, but it keeps multiple bag windows, which I don't like as much, having used AllInOneInventory/OneBag/Bagnon and the like pretty much since I discovered mods.
Engbags at Curse
Advanced Bags Plus at Curse