I know free poker sounds like a good idea, but it's not.
Let's say you know how to play cards, but you want to improve your game before you put any of your hard earned cash to risk on online poker sites. It's a great idea, in theory, but playing without spending will certainly make your game worse. The yahoos in play money rooms will see every bet and raise randomly with any cards if they haven't already gone all-in preflop. Bluffing is impossible, so improving is impossible. Not everyone at the table may go in with the intensions of making a mockery of poker, but everyone will start eventually just to keep up...including, probably, you.
Let's say you never heard of texas hold'em before and want to learn the rules. Playing for free can't hurt then, right? Wrong! You want to learn how to play, ask a friend to teach you. With absolutely no incentive to win you are not learning poker, you are picking up bad habits. These will stick with you for a long time. Learning right first is learning right best.
Let's say you are a hold'em expert and you want to pick up omaha. You know how free poker works and are confident you can avoid the insanity. That may well be, but I still wouldn't risk play money games. Switching back and forth from from cash poker and free poker is very confusing. You might forget who you are playing with.
If you want to learn how to play poker: make the investment. It doesn't take much. .05/.10 cent blinds is how I started out. I was with college students who had to make five bucks last the whole week, so they valued their coins. You just have to find the sweet spot to keep out most maniacs. If you prefer tournament play, I recommend $10 buy-ins with no rebuys for solid learning. $5 is okay, but you get plenty online that don't take five dollars seriously.