A relatively new private server has launched, and it even has a web client, which means that no download is needed! Really amazing how private servers are progressing.
Here’s an image of the web client (click to enlarge)
What’s also neat is that they can change locations, like the ice rink could be during the summer, and the dock could be changed to when it was during Christmas.
Register here: http://clubpenguinv.com/blog/register/
And play the private server here: http://clubpenguinv.com/play/
Also, I have not confirmed that the passwords are encrypted, so just to make sure, remember to use a password you have not used before!
NOTE: Their servers are often online and offline – they are working on a better uptime, in the meanwhile, if the link does not work, check back later.