LTC is the big, popular altcoin - currently worth about $18/LTC
I, personally have been trying to get involved in Datacoin (DTC) - it's a newer coin and isn't on many exchanges just yet, but that's in the works.
Another one I thought looked good is Diamond coin (DMD) - they were having a lot of server/network issues the past few days, but it's likely due to the fact that they're reviving a coin and probably didn't expect the response they got. I've heard things have been smoothed out for the most part now.
This is all just my opinion, so you can take it with a grain of salt. Happy mining!