TooManyItems

TooManyItems is a mod created by Marglyph. The mod allows the player to visually choose and get directly in his inventory any object (blocks or items) from the game without the need to type any command like "/give " (unlike INVedit, which requires going to the menu to edit your inventory), including acquiring infinite stacks of items, blocks and infinite tools. As well as this, it also allows you to save up to seven different inventories and even throw stacks into the trash when they are not needed. The mod is currently compatible with Beta 1.2 through 1.0.0. The mod needs Mods/Modloader to make unlimited stacks of blocks refresh automatically. Most recently, buttons at the top left corner of the inventory screen have been added. The buttons can switch your game mode from survival to creative and back, change time of day, turn rain and snow on and off and delete your entire inventory.

Trivia

 * In SSP, if you shift click on armour items, as an attempt to get infinite armour, and then equip it you will receive absolutely no damage from anything (with the exception of the void) even after the armour if removed or thrown away in the trash.
 * In 1.0.0 now you can change from creative mode to survival mode. You can also change from day, midday, afternoon and midnight.
 * You can hide the TooManyItems box and information by pressing 'O'.

Download
The file can be downloaded from the forums here and has compatible versions for Beta 1.2 through 1.0.0.