Gunbound: World Champion. Three words--Fun. And. Addicting.
Gameplay: You enter a room, have a choice of 16 bots (and 2 secret bots by picking random), have a list of items to choose from, and off you go into the game, to try and kill your opponent faster than he can. Gameplay consists of score, where each team has one more life than number of players on their team; solo, where, if you die, you don't come back for the rest of the game; tag, where you have one bot at full health and another at half health, but if you die, you don't come back for the rest of the game; and finally, jewel, where you aim to kill little jewels with numbers above their heads, if you kill it, you get said number of points, first to 100 wins.
Graphics: Very cartoony, but still clean and fun to look at, all the characters have big ol' eyes and customizing your character with Avatars can be really fun.
Sound: Very well done here, all the shots, explosions, thwacks, bams, ker-pows and whams can be heard quite clearly. Music is a round-up of about 10 different MIDIs that are enjoyable to listen to.
Value: If you download this game, you'll thank yourself later. Though some items in the game do require you to have a credit card to buy them (don't worry, it's completely optional), the rest of this game is 100% free.
Tilt: Warning: EXTREMELY addicting. Once you start, you may not be able to stop. This game is that fun.
One of the downsides to the game is that if you are really bad at it, people will call you names and insult you, and if you're really good at it, they'll say you're hacking (aimbot is an auto-aim system used by hackers), call you an aimbotter and usually will kick you from the game. Also, sometimes finding a game can be hard since most of the time everyone on one side of the team are really high ranks, and they won't kick the low ranks that shouldn't be in that server out. It can be frustrating at times, but having friends to play with makes it all the better.
GunBound World Champion is a great game and I highly recommend it if you're wanting to basically give up your whole day for a game that's just so darn addicting.