The iPhone has brought a lot of unique features to portable gaming -- a large, multi-touch screen, a tilt sensor and significant built-in storage among them. But one of the iPhone's most revolutionary features is its large library of instantly accessible free games. Every previous portable system required at least a little bit of money, effort and forward planning to buy a game and take it along, in anticipation of future gaming on the go. Now, with the iPhone, you can download any one of thousands of free games the moment you find yourself with some free gaming time.

The question, of course, is whether any of these free games are worth your time. To find out, I thought I'd put the App Store to the test by downloading the top 25 free titles from the "All Games" category (based on number of recent downloads as of July 17, 2009), and playing each one for as long as it could hold my attention. The only caveat was a 15-minute time limit for each game -- because if a free game holds your attention for at least 15 minutes, that's pretty good!

With that said, let's dive right in to the top free games:

1. "Wooden Labyrinth 3D Lite"

I had a love/hate relationship with my old tabletop "Labyrinth" game as a kid. I loved the concept, but I hated how much I sucked at manipulating those little knobs to guide the ball past the holes. The iPhone version, with its simple tilt controls, makes things more enjoyable. I'm surprised how accurate and realistic the physics is, and I love the way the 3-D perspective shifts as you tilt the unit, making the screen feel like a window into an alternate world. The randomly generated levels in this "Lite" version are hit-or-miss, but the variable difficulty ensures lot of replay value.

Time wasted: 15 minutes

2. "Assassin's Creed: Altair's Chronicles Free"

The sprawling next-generation experience of "Assassin's Creed" takes quite a few hits on its way to the iPhone. Notice the blocky, grainy 3-D graphics and nearly empty streets. The gameplay has been also simplified to the point of stupidity, with a big green arrow pointing the way, ridiculously frequent checkpoints, and battles that amount to jamming the attack button as fast as possible. Add some horrible voice-acting and writing (Actual quote, from Altair: "I should kick this post so the platform will collapse!") and you get a game I was happy to put down when the "Thanks for Playing" message stopped me at the 10-minute mark.

Time wasted: 10 minutes.

3. "Rope'n'fly Lite"

"Touch on a building to throw a rope. Touch again anywhere to release the rope. Try to swing and fly as far as possible before crashing on the ground." Simple enough, but in practice the physics of swinging seem just so slightly off. Flying through the air is fun, but the rope has a bit too much slack, and the swinger will often hang in place at weird angles if you don't time the swings just right. It only took me five minutes to survive the Lite version's 30-second timed mode. I could try to best my own high score, but I don't find myself exactly relishing the thought.

Time wasted: 5 minutes.

4. "Tic Tac Free"

It takes me one try each to beat the iPhone on Easy, Medium and Hard modes of this barebones tic-tac-toe game. I wonder how many of the doubtless thousands of downloads of this game were deleted within five minutes.

Time wasted: 1 minute.

5. "Paper Toss"

Only after I load this up do I realize I used to have a minor addiction to the Web version of this game. Throwing a wad of paper into a wastepaper basket is made surprisingly addictive by variable wind speeds and a streak-based scoring system. (The satisfying "clang" sound when you swish a shot might have something to do with it, too.) The iPhone version has some nice, flick-based controls and a clean, simple interface. While Easy mode was pretty simple to master, and Hard mode is way too tough, I had a great time wasting 15 minutes on Medium.

Time wasted: 15 minutes

6. "World War"