This simple mod adds an icon to the player's HUD at the bottom right corner. It shows how many inventory slots are available and how many of them are occupied.
This mod also supports the Bags and Unified Inventory mods. If one of these mods are used, icons for each bag are displayed, too.
Simple. yet elegant
Definitely recommeding this mod, now I don't need to check my inventory and keeping in mind how much items I can carry around :)