You can pick up a N-Gage for little money nowadays, and there are some great games for this system! Pocket Kingdom is nicely addictive and has multi-player options.
First, Pocket Kingdom bills itself as being the first MMORPG on a handheld. It's amazing to think of any handheld game as being MMORPG capable. On the downside, in most areas there are few N-Gage players to hang out with.
You can make several parties of 4 characters, starting with the basic warrior or mage types. As you roam around maps, you can send various parties into battle. The characters then auto-fight based on their abilities. You sit and watch, hoping that you equipped them well enough to defeat their enemies. In a way this makes more sense than a high level character's defeat being caused by poor reaction timing of the person playing the game.
Each character can wield one special item to help boost their stats. Instead of finding weapons, you find "molds" for weapons and have to track down the components. There are hundreds of items available for use.
You can choose which types of battles to enter, so you can hang out in easy areas to boost your stats, or head into more difficult arenas to face a challenge. It's pretty easy to get in a few battles while you wait in line, pause it, and pick up the game later on when you have time again.
The game tries to be L33T in its gangster slang, which is at times funny and at times a bit silly. The sound has simple soundtrack backgrounds, and the graphics are simple but effective. This certainly isn't a game that holds up to PSP standards, but on the other hand the cost is incredibly less. I picked up this game for 1/10th the price of most other games on the market.
Well recommended for any N-Gage owner who enjoys RPG adventure.
Buy Pocket Kingdom from Amazon.com
UPDATE: Unfortunately, the N-Gage unit and its services were discontinued in 2010. It became a victim of the modern Android / iPhone systems.