If you have a PS3 and you haven't downloaded any demos yet, I think you're missing out. There is a bunch of free stuff that can keep you entertained for hours. Here is a list of some of the demos that I've played. If you have played some PS3 demos, be sure to tell us about them here.
Bomberman Ultra is one of my favorite demos... simply because it's pretty much the same game that you'd get to play if you bought the full game. Ok... so it's lacking quite a few features, but that's what demos do. But at least with this demo, you can play it over and over and over. I have the old PS1 Bomberman game, and the standard game was about all we played anyway, other than choosing different maps sometimes. There are a couple maps that you can play on, and at least two different game modes, they even allow some character customization if you want to spend the time customizing your Bomberman to look like a pirate or ballerina. Games are quick, and it looks like you could have up to 4 human players on at once. (The full version allows 8 players at a time - sounds crazy... and that's a heck of a lot of controllers to buy!)
Comet Crash is another favorite at our house. It's a "tower defense" game, where you and your opponent each have a base and you're primarily building turrets around to defend them. You also can make "Ops" buildings to get troops which you can send out to attack the enemy base. With two controllers the demo game works for 1 or 2 players in Training, Campaign, or Battle modes. Training teaches you how to play step-by-step. Campaign pits the humans against on AI opponent. and the Battle pits the human players against each other. There are multiple difficulty levels for the Campaign which can all be used, but the demo stops at 3 levels. This is another fun demo, with plenty to play for free!
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Now that I've mentioned a couple of my favorites, I'll mention one demo that is not really worth the time to download... except maybe just to remind you that you don't need to buy UNO for the PS3. Oops, I gave it away...
UNO is probably the biggest waste of effort for a demo. You get a maximum of 10 minutes of playtime - 5 minutes in Single Player mode and another 5 minutes in Multi-Player mode. Of course, we all know what UNO is... even my kids know how to play UNO, which is why my son asked me to download the demo. So he got to play one game by himself, maybe two, before it timed out and went to a BUY ME screen. The next day he asked me to play it with him, so I did. We were able to finish one game. A couple other bad points about the UNO demo, which would surely be the same in the full game, are that your cards are visible to everyone else looking at the screen, which might be only briefly if you're just hitting X as soon at it's your turn (the highest point card for the color appears to automatically be chosen for you) but in that time someone COULD potentially see what colors you have to block your win... which is one of the few strategic components of UNO. The second, and probably worst, problem that I see with the UNO game is that if you don't "Call UNO" by hitting TRIANGLE before you choose a card, you can't "Call UNO" afterward... you just wait for someone to "Challenge" and draw your penalty cards.
That's about all the time that I have to discuss demos for now. Let me know what demos you've played, which ones are great and which ones aren't worth the HDD space.