Identi.ca

This is a new one, its a website. If you have ever heard of Twitter.com, it is modeled after it. It is essentially a micro-blogging service. What micro-blogging is basically updating what you are doing. For example you are hiking down by the local river, you can explain that in 140 characters or less.

"Identi.ca is a micro-blogging service based on the Free Software Laconica tool." - from their website.

This means that the website is not really open source, but the software, laconica, is. It is built on PHP.

Check it out at identi.ca.