WordPress MU is the base of the code is used on WordPress.com, and allows anyone to publish their own hosted blog service quite easily.
This may not be entirely related to web 2.0 developers, though its use of ajax and the resulting functionality justifies an entry here on builder2. AjaxAMP is a Winamp plugin that allows you to control a Winamp player in a remote PC, and more importantly, access your music library from anywhere on the web!
Basically, this is music remoting and shoutcasting taken to a higher level. This is not about music piracy or illegal copying of music, but rather listening to your library whenever you need it. I’ve tried the plugin and it works like a charm, though there is still room for improvements. You’ve got to actually try it to appreciate its usefulness.