I did a search and couldn't find any mods like this, so I made it myself: a mod that increases the size of your inventory. It adds another row of slots, totaling 45 slots excluding armor. Eventually I'll add the ability to configure it for however many slots you want.
Most modded items in the image are from More Ores, and the banana is from More Trees.
It modifies the classes bf, bw, gf, hc, ie, ih, nr, nw, re, ta and y. It does not depend upon anything and can be installed alone. It is compatible with TooManyItems and ModLoader, and should be compatible with most mods that only add blocks or items.
Unfortunately, it may not be compatible with mods that add blocks which have their own interfaces, such as IndustrialCraft. When I figure out how to work ModLoader, hopefully I'll attempt to fix this.
Because it adds slots, it will not appear correctly with texture packs. It will still operate properly, however.
Installation on Windows:
1) Click Start Menu, click Run and type %appdata%.
2) Browse to .minecraft/bin.
3) Open up minecraft.JAR with WinRAR.
4) Drag and drop the class files into the JAR.
5) Drag and drop the gui folder into the JAR if you wish to make it default. This folder can also be used in a texture pack instead.
6) Delete the META-INF folder in the JAR.