It's proven to be stupidly popular, so check it out here and see for youself.
There are a a few different modes, some which are free to play and others where you need a paid account.
Notch's development blog
The Minecraft Wiki
The Minecraft Forum
#minecraft The MineCrafters' group chatroom.
Apply to the group via note to have your eligible server listed here.
SMP: legendsmc.dyndns.org | Contact: jgnatiuk | Website
SMP: smp.mcsteamed.net | Contact: youlikethaaaat | Website (1, 2)
SMP: 188.8.131.52 | Contact: BOBSYERUNCLE11 | Sexpot website
SMP: aeonianrealms.com / aeonianrealms.com:1010 | Contact: Andream0219 | Website: AeonianRealms
This is not currently an all-inclusive list just yet, but we will be adding items to it as we go. It will list suggestions to try and guidelines to follow when submitting to the group.
Pay attention to the name of the group folder you elect to submit to.
(If you have to remember only one thing about these guidelines, remember this one. Every incorrectly submitted deviation detracts from the enjoyment of running the group, which is key in maintaining motivation to process incomming sumbissions. It also makes things take longer.)
The In-game Screenshots folder is for in-game screenshots. Avoid posting your drawings and manipulated screenshots to this folder. A sensible level of overlaid text on a screenshot is still counted as a screenshot.
When submitting drawings using screenshot backgrounds, use the Non-Screenshot General Art folder.
Avoid submitting screenshots to the Non-Screenshot General Art folder.
When submitting to the Downloadable Player Skins directory, avoid submitting only the flatpack image or linking elsewhere to another site or deviation for the download. Place the flatpack image in a zip file and use that as the deviation file and set the deviation preview image to display the constructed view. Do not submit two deviations of the same skin.
Do not submit multiple instances of the same deviation because of processing delay. It adds unneccesarily to the workload and is not effective in any case.
Do not advertise on the group wall. Note the group if you want to apply for group listing.
Please do not advertise on the group wall.