Minecraft In-Game Inventory Editor
INVedit is a wonderful inventory editing tool, but it can be a little inconvenient to have to stop playing the game, save the file and then open the INVedit program and load the inventory of the particular save you're playing, edit it, save it again and then restart your game. Too Many Items is an inventory editing mod that can be accessed in game and toggled on and off with the 'o' key. When the mod is toggled on, opening the inventory screen will give you the overlay below.
On the right hand side of the screen is a list of all the available items in the game. At present time there are eight screens that can be clicked through in order to find the item you want. Clicking on one of the icons will add one icon to your inventory. Holding down shift whilst you click on the icon will add an entire stack to your game.
Too Many Items also contains a 'fast craft' option that allows you to right click in order to create a whole stack of a crafted item. No more clicking half a gazillion times to get enough stones to pave a road! No more developing RSI in order to build structures of amazing magnitude!
Another useful function is the 'trash' command. You can drop items from your inventory onto a trash icon to get rid of them. Right clicking and pressing 'delete all' will clear your inventory completely. This is handy for those times when your inventory gets full of all sorts of rubbish accidentally picked up in the course of mining or when you finish a project and don't need the leftover materials.
It is also compatible with some multiplayer servers, but you must be an OP on the server in order to use the mod, and chances are that any server you join as a guest won't OP you.
This mod can be installed via TFC's Mod Manager, an independent interface that allows you to install mods without messing with the minecraft.jar directly yourself. It's also fairly easy just to drag and drop the files into the .jar and be done however.
It is possible that you'll find some incompatibilities with other mods. As usual, that's very much the luck of the draw and only trial and error will show what mods work together.