Litecoin is becoming a very strong contender to Bitcoin [1]. It will soon be on Mtgox, according to their press reports. Feathercoin has also picked up steam, and it's interesting because it's brand new and already popular. Namecoin, which has been around since 2011, is still a strong contender with long term potential and value. The main difference between Litecoin and Bitcoin is the hashing algorithm used. Litecoin also has faster transaction confirmation times, since blocks are mined at a 4x rate. In my best opinion, Litecoin is better technology than Bitcoin.
I would have written a lot more detail, but I can't trust people on HN to upvote my post, and it would be a wasted effort. Just listening to myself gets old quickly...
Anyways it is a worthwhile thing to look into these before the mainstream gets to them. There are two main exchanges to trade alternative cryptocurrencies, here [2] and here [3]. Also there is a forum where they are discussed and you can monitor the hype [4].
[1] Litecoin is now accepted for donations on The Pirate Bay, the number one torrent site
[2] btc-e.com
[3] vircurex.com
[4] bitcointalk.org/index.php?board=67.0