An online collectible card game, part Magic:TG and Pokemon. Buy sell and trade fighters, train them up and fight your rivals.

A neat game that is free, quick to register for, and quick to learn. Tricky to master though. Addictive.

Urban Rivals

A civilization-type game where you build up your village, build troops and take over other villages.

Takes some time to build up and master, but it will keep you entertained for months, and give you something to do at work when you have time to build some buildings.

Tribal Wars

I was going to write a tutorial for getting started, but I didn't get a round tuit.

Aha, I've finally gotten around to my Tribal Wars help lens.

Pick a class, roam around the extensive map, and kill zombies. Or become one and prey on the human players. You can communicate, barricade yourself in and fight other players.

Urban Dead

Puzzle Pirates

Go from island to island, buying better gear and fighting other players and ships in a variety of puzzle games. (My favorite is the Puzzle Fighter-esque Tetris game).

As you progress, buy better pirate togs and gather gold.

For you gear jockies, try Truckz.

Drive from town to town with the hammer down, shipping loads of goods, keeping your truck up to date and not getting pulled over by the county mounties. Like Tribal Wars, its addicting to keep up your truck and buy new parts.

Truckz

Created in 1998 by Tom Fulp, what started out as a showcase for his own work became a popular place for others' work as well.

He created the Pico series and Alien Hominid, which eventually became a console game in its own right. He is following it up with Castle Crashers.

Meanwhile, the Internet denizens have turned out to create and showcase their own games.

In addition to thousands of games, they also have user submitted flash movies.

Newgrounds

These guys are very prolific, and very talented. I'm always getting stuck here playing games for far longer than I should, which is fine!

Armor Games

Boomstick - Scroll side to side while firing your shotgun at random shapes. Good hand/eye coordinator.

Protector - A Tower Defense game with wizards. I like TD games.

Compulse - A fun physics/gravity game. Use the tiles to direct your ball to the goal.

Portal, The Flash Version - Not as full scope as the real thing, but a remarkably sophisticated knock-off. The cake is a lie.

Shift and Shift 2 - Portalesque game where you shift the world to get around obstacles.

Momentum Missile Mayhem 1 and 2 - Direct your cannon to destroy the onslaught of tanks. Shooter meets pinball.

Four Second Frenzy - A bunch of intuitive mini games that you have 4 seconds to complete before racing to the next one. Crazy.

Music in Motion - Run and jump, don't let the edge of the screen catch you. Like being in your own techno action music video.

The Last Stand - Shoot zombies, defend your stronghold, search for guns and help. A lot more than just a zombie shooter.

S.T.A.R. Defense - Build satellites to save your planet, part shooter, part TD. Nice variation.

DS Flux - Nice Space Shooter but you only get one life.

Biolabs Outbreak - Isolate viruses and kill them off in this Qix variant.

Indestructo Games - Use your vehicle as a weapon and kill everything in your path.

Surrounded - Neat space flyer/shooter. Upgrade your ship and survive.

Pixel Fighter - Vertical space shooter, 500 levels with weapon upgrades as the tunnel gets smaller and smaller.

Bloons and BloonsX - Use your monkey to shoot bloons with darts. Special bloons help you beat tricky puzzles.

Horse Run - Get your horse to jump over the blocks. Tricky.

Coffee Shop - Run a coffee shop, a good variation of the old Lemonade game.

Contour - Get your ball to the goal using nothing but the terrain.

Agnry Faic 1 and 2 - Get as high as you can by squashing the other faces.

Skywire - Okay, this is a high quality game, reminiscent of some NES games. Get your little cable car to the end of the track. Cute.

Bloons Tower Defense 1 and 2 - If you like Bloons and TD, then you'll like this. Build monkey towers, destroy the bloons.

Bot Arena 1-3 - Build battlebot type machines and duke it out in the arena. Select chassis, armor and weapons.

Turret Defense - A mission-based TD.

Elemental Turret Defense - A nice variation, build different towers, and some buildings to improve your setup. What's different is you have to power the towers by toggling the arrow keys. Keeps you from setting and forgetting.

Galactic Gravity Golf and Deluxe - the best gravity games. Get your ball to the whole, hear the crowds go crazy.

Deluxe Pool - The best 9-ball sim I've ever seen. Shows angles, two players.
X-Hoc - Soccer with little robots.

Base Jumping - Land your little blob first, without splatting him to move onto the next round.

This is just a fraction of the games available. Check them out for more fun.

Another repository of flash games and the like made by them.

Miniclip

Slingfire - Control your blob by slinging him to other points, each with different effects and physics. Addicting leveled puzzle game.