What Bitcoin-esque alternatives are there? I'm seeing lots of bitcoin-esque alternatives mentioned here. What alternatives exist and how do they differ from Bitcoins?
There are a number of other cryptocoins in use alongside Bitcoin, the most notable are:


Testnet Bitcoins (used only for testing Bitcoin applications, but they are an important mention) - they work exactly the same as Bitcoins.
Namecoins - coinage used as a distributed naiming system blockchain. Their most notable feature is that they can be merged mined with Bitcoins.
LiteCoins - "silver to Bitcoin's gold" - their blocks are generated every 2,5 minutes, and their hashing algorithms are more CPU-friendly (mining with graphic cards doesn't work well with those).
SolidCoins - 2 minute blocks, there isn't a limit on how many coins will be generated, CPU friendly, difficulty is adjusted more often, close-source. The SolidCoins have undergone a "reset" recently and have earned themselves a negative reputation for how some issues were handled.
GeistGeld - 15 seconds blocks, making it the fastest alternative crypto-currency.
I0coin - they don't list any difference from Bitcoins on their website
Ixcoin - same parameters as Bitcoins, asides more coins being generated each block (thusly, all 21M coins will be generated sooner)
Tenebrix - CPU friendly, a few million coins pre-minted


A list of them is also maintained on the Bitcoin Wiki.

There is a relative ease in creating new cryptocoins, as you can just tweak the main client, encode a new genesis block and start your alternatives with small effort.