A very large majority of these altcoins are essentially scams. There is a cottage industry that takes a coin, changes something minor (name, logo), and releases it with great fanfare, enticing people to buy it, allowing the forker to unload their typically worthless coins.
As Нонн Болдырев said, there is a small list of worthwhile coins. I'd leave Ripple out, since it's not a cryptocurrency per se, but otherwise his list seems sensible.
Here's my very short list of interesting currencies:
- Monero: adding privacy and fungibility (the most prominent of Cryptonote coins)
- Ethereum: smart contracts, ability to create contracts on the blockchain, automatically redeemed when predefined conditions are met
- Cryponite: mini blockchain, solving the scaling problem (though it's not being worked on anymore)
I can't comment on the others on Нонн Болдырев 's list due to not knowing enough about them.