Too Many Items is inventory editors that also has some extra features.
On the inventory screen, all items installed in Minecraft and installed mods will be displayed on the left. Thus, allowing you to use every item you ever wanted as you please.
This also includes:
Time (Day, Noon, Midnight, Dusk)
Difficulty changer
Rain/Snow On/Off
How to use its features:
Turn On/Off: Press the "o" key in the inventory screen. By default it's on in single player and off in multiplayer. On/off state will be remembered. You can change the key (to a different one than "o") in the config file (see below).
Add items: Left-click on items in the right sidebar to add full stacks, or right-click to add one at a time.
Configuration: Find the directory in which your minecraft saves folder and options.txt are, and if you have previously used TMI you will also find TooManyItems.txt there. This contains options that you can change, not all of which are documented yet. (Important: only set spawner to the name of a mob, like Skeleton, Sheep, etc. If you set it to another entity like a boat, your game will crash.)
Unlimited stacks/tools (single-player only): Shift-left-click on items in the right sidebar to add unlimited stacks OR unlimited-use tools or flint and steel. ModLoader is required for "true" unlimited stacks. Without ModLoader the quantity of >64 stacks will refresh when you open your inventory.
Trash (single-player only): Drop an item stack on the item sidebar or the trash icon to delete it. Click the trash icon to turn "delete mode" on and off (in "delete mode" you will delete any item you click on -- useful for selectively deleting a lot of items). Shift-click thee trash icon to wipe out your inventory.
Toolbar: Next to the Trash icon, there are icons that turn on/off creative mode, turn on/off rain, and set the time to dawn, noon, dusk, and midnight, respectively. If you don't want this functionality in your TMI, you can set itemsonly:true in your config file. Note: in multiplayer, these buttons send commands to the server and you'll need to be an op/have the appropriate permissions. There is currently no way to customize the commands that are sent.
Save states (single-player only): Several slots are available for you to save your entire inventory and restore it later. (The "x" button next to a saved state will remove it.) This can be used to save your "real" inventory before editing, save a blank inventory to clear out everything you're holding, save a full inventory of materials, share inventories between characters, etc.
Multiplayer: You need to be a server op. Also, every non-vanilla server works a little differently. The command issued to the server to give you items is configured in TooManyItems.txt as "give-command". In this command, you should put {0} for the player's username, {1} for the item ID, {2} for the quantity, and {3} for the damage value. If {3} isn't in the command, items with damage values other than 0 won't show up.
Vanilla server command: /give {0} {1} {2} {3}
* Since the vanilla server used to not support item damage with the give command, TMI will probably still have /give {0} {1} {2} by default. You can now add {3} to the end. The default will be updated in a future TMI version.
Bukkit with the Essentials mod command: /item {1}:{3} {2}
Fast crafting: Right-click on the output square when crafting to craft the maximum possible amount. (Works while the inventory overlay is disabled).
Fast transfer (single-player only): Hold shift while placing an item into a chest, or into your inventory from a chest, to transfer all items of the same type and combine stacks. (ConvenientInventory does this better, and TMI will use its functions if it is installed. Make sure to ONLY install ConvenientInventory.class).