Ok there is afew online games that you can hack that arn't too bad (nothing special but better than nothing.) Check this site out http://www.maidmarian.com Sherwood and Tank Ball are properly the best games on there and you can hack them using CE or any other type of memory editor. You will need to install Shockwave to play them.
condor and wr194t your signatures are too big, you need to cut them down (vertically)... However, hacking an HTML game is simply not possible (if you can even find one), also hacking PHP games can be hacked, but a well secured PHP site may prove quite a challenge. Easiest way upfront is SQL injection, manipulate a variable into positive effects... It's tricky no doubt, so do some research on it before you try it.