Unlock Moria and Helm's Deep Bonus Levels:
When you beat the game with any character, you unlock both of these bonus maps, which can be accessed on the main title screen.

Moria - Kill 2000+ orcs with one character
Helm's Deep - Beat the game with any character

Unlock more bonus levels:
You can unlock Weathertop by collecting all of the artifacts. You can unlock Fanghorn by beating the game with all of the characters.

Weathertop Bonus Levels - Collect all of the artifacts
Fanghorn Bonus Levels - Beat the game with all of the characters

Unlock Sam:
Sam - Link your Game Boy Advance ROTK game with your GameCube ROTK game

Ancient Artifacts:
To unlock the ancient artifacts on your Game Boy Advance game, you must connect your Game Boy Advance Return of the King game to your GameCube Return of the King game. Ancient artifacts' names are in blue, but they are not unique items. Actually, they are fairly common once you unlock them, and some of the ancient artifacts have extremely good stats.

Unlock Smeagol:
Beat the game with any two of the starting characters

Duplicating Items:
(NOTE: You'll need at least one blank save slot) To duplicate good items that you find in your quest, just:

1) Get to a level with any Forge or Shrine and a Ranger Hollow.
2) Have the item(s) in your inventory.
3) Save the game as normal, then save it to all the blank save slots you have.
4) Quit the game, start up one of the games in the save slots you just filled, and put the item(s) in the Ranger Hollow of the level.
5) Do this with all the save slots you filled.
6) Go back to your main save, and take all the items out of the Ranger Hollow, you know have duplicates of your item.
7) Repeat as wanted.

Get Sam without a Gamecube:
Many people like myself don't have a Gamecube or GBA-GC Link so here's how to unlock Sam without one. First you may want to beat the game with Frodo to get all of the artifacts in his quest so then if you have all of the artifacts you can play at Weathertop with Sam. Then start a new game as Frodo. Make it through Shelob's tunnel and Shelob will ambush you making you have to use Sam. Save the game during the fight with Shelob then quit. Now you can hook up the Gameboy Advance link cable with Sam or play on bonus maps with Sam. This could also be used for training Sam against Shelob if your having trouble.

Infinite Stat Points:
The the map should be 'Anduin River - Corsair Camp'. Talk to the old lady at the north of the map to do the Missing Brother Sidequest. After you return her brother, talk to her again, she'll drop a random item and you obtain 1 stat point as well. Proceed south from here, you'll meet the old lady again. Talk to her and defeat the Corsairs. After that, she'll open up her barn (or ranch), then talk to her and she'll drop another item. Save, quit, reload and repeat. (Note: If you didn't talk to the old lady after she opened the barn, when you reload, the game will know you've finished the missing brother sidequest, which means you can never get the 1stat point reward from her again.)

Massive Damage and Armor Boosts:
Go get a character that can equip a two-handed weapon, and a shield. Then, equip the sword and shield alternatly, and you will notice your damage on your stat screen going up. Carve armor runes to make your armor level rise too. This works with Eowyn, Gimli and Aragorn. The bug will reset once you leave an area.

Duplicating Items in Multiplayer:
If you are playing in multiplayer, there is a simple trick that you can use in order to give items away to other people without losing the item yourself. When you are in multiplayer, choose an item and give it to the other player. Then, quit the game without saving. Have the other person save and quit the game. When you reload, the game will not have recorded that you gave away the item so you will still have it. Since the other person saved, they will also have the item.