I thought it might be a good idea to make a list of short, one-sentence descriptions of the most important altcoins. This idea isn't new, but most of the existing lists out there are old and outdated. Of course, it's possible to search the name of each coin on Google and go through the links one by one or read through each coin's [ANN] thread but this can be a rather tedious process. So I think this would save people some time.

Since the list is always changing, I'll try to update it every few days/weeks.

The coins in red are ones that I don't know much about, so they currently lack a description. I might add their description later on. Feel free to suggest a short, one-sentence description and I'll add it to the list. If there are any mistakes or I've left something important out, then feel free to point it out as well.

Bitcoin: The original cryptocurrency
Ethereum: Decentralized application platform for running 'smart contracts'
Litecoin: First successful scrypt coin
Ripple: Distributed P2P currency-agnostic payment network
Dash: Anonymous coin with instant transactions and decentralized governance
Monero: Bytecoin clone (CryptoNote-based anonymity) with fair distribution
Ethereum Classic: Fork of Ethereum where the hacked TheDAO funds were not returned to investors
NEM: NXT-inspired coin with modified PoS system ("proof-of-importance") and wider initial distribution
Augur: Decentralized prediction markets built on Ethereum
MaidSafeCoin:

Golem: Ethereum-based decentralized computation network for buying/selling computing power
Zcash:
Tether: Blockchain-based representation of the US dollar backed by fiat reserves
Dogecoin: First meme-based coin
Decred:
PIVX: DASH clone
Stratis:
Waves:
Factom: Blockchain-based record-keeping system
Steem: Reddit-esque social networking/blogging platform

Iconomi:
DigixDAO:
BitConnect:
SingularDTV:
Round:
Lisk: Decentralized Javascript application platform
First Blood:
Bytecoin: Anonymous coin and the first to use CryptoNote (ring signatures)
GameCredits:
BitShares:

Miscellaneous coins not in the top 30:

Auroracoin: Coin for Icelandic residents
Burst: Coin mined using free hard drive space ("proof-of-capacity")
Gridcoin: Coins rewarded for contribution to BOINC network
Huntercoin: Human-mineable coin where coins are harvested by playing a game
Namecoin: Decentralized DNS (Domain Name System)
NXT: First pure proof-of-stake coin
Omni: Platform for creating custom assets/currencies on top of the Bitcoin blockchain
Peercoin: First coin to use proof-of-stake (actually hybrid PoW/PoS)
Primecoin: Coin with proof-of-work system that searches for prime numbers
RISE: Lisk clone