If you know C and C++, you should be able to make your own alt coins! 
https://bitcointalk.org/index.php?topic=225690.0
https://bitcointalk.org/index.php?topic=225690.0
... but please don't, there are quite enough out there already thank you.
Most of them are unique. The thing is that only a few are unique:Bitcoin is original
Litecoin is the first (not 100% sure about this) scrypt coin
Quark coin has multiple algorythms to support it
All of these are VERY successful alt coins! All you need to make a good coin is a unique idea!