Where to paper trade?
You might check out TOS. I think they have an awesome virtual system.
I used that as well however for me I didn't have the emotion with paper money that I did with real money, no matter how hard I tried with paper the emotions just weren't there. I pretty mad had to practice with cash.
I use Investopedia.com's market simulator. It's probably not the best, but serves my purposes pretty well.
(blinks also raises a good point; simulator's don't take into account emotional instability when trading with your hard earned money. Though they're fantastic for the "what if?" stocks that you might not invest cash in.)