Indenti.ca is a new microblogging service with extra features that immediately place this as a potential Twitter killer: it's open source AND it supports OpenMicroBlogging.
If OpenMicroBlogging means nothing to you, let me explain. From very early on folks like Dave Winer have been calling for decentralized microblogging, where you can host your own microblogging platform but each site could communicate with each other. The main advantage of this system: when one service goes down the whole system doesn't fail, like it does with Twitter. If you weren't getting good service with one provider, you could then easily switch to another provider, while keeping the same friends and identity. Competition in this way encourages excellence, as opposed to Twitter which really only encourages lots of moaning.
The release of the code behind Identi.ca (called Laconica) is a bonus again, as in theory anybody can grab this code today, install it, and run their own microblogging service that is cross compatible with Identi.ca.
Bwana points out in a post that Indenti.ca is fairly feature poor so far, and the service lacks basics such as an API. There is a long list of "soon" features here, the bigger question though is how quickly some of the key features can be rolled out to keep people using the service?
Dave Winer on the other hand simply headlines it as "Oh happy day." This isn't a perfect service, but with open source code and cross host microblogging, Identi.ca could be the first real step to decentralized microblogging, and given Twitter's ongoing service crisis, could be the beginning of the end for Twitter.
If you want to follow me (and I'll follow you back) I'm on Indenti.ca here.