Armorgames usually has a decent selection. Kongregate as well, and Newgrounds. You'll find tons of interesting games at those sites.
IMO the key is to pick new games that people may have not played before that are, at the same time, decent and challenging. If you choose to pick old games, then at least pick ones that have some rave value to them (e.g. Chip's Challenge, or any other game that many people have played and loved over the years). Mindless games or games with a ton of repetition aren't very fun and don't really provide incentive for competing hardcore-style. Also, try to steer clear of games that have clunky controls or hit detection. Not being able to finish a game because of a glitch or crappy movement scheme is maddening. Stick to higher quality stuff.
I tried to pick really good games for my first Omnipwn tournament. Check that out if you want an idea of how I hosted it. My second tournament had limited participation and I wasn't as excited about it, and as a result, my game selection was very half-assed and apathetic, as I didn't take the time to choose interesting games like I did beforehand. Of course, I'm merely speaking for myself here, but I think a lot of people had fun in the tournament and enjoyed the game selection in the first Omnipwn tournament (at least from what I see -- if anyone disagrees, please let me know, lol).
If there's anything I learned from hosting a tournament, it's that you really have to be active and excited about it. You have to update often, and you have to put some decent effort into looking for interesting games that you yourself would love to play and whore out in a competition. If any of those factors is at a low, then the tournament tends to lull and fall apart.
On that note, I hope to have more time this summer to really run a decent tournament. It takes a lot of time, imo.
Java is all that is needed for this game. I got my computer fixed, and reset itself to warehouse default, so I needed to re-install Java last night. It should work. If not, can't help you. Sorry.
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