From Phillip Pearson's Python Community Server Blog (of all places):
While everybody has been talking about Chandler, the PIM Mitch Kapor's Open Source Applications Foundation is planning on developing, Diego Doval, the guy behind the 'blognovel' Plan B quietly announced his own spaces, which looks far less like vaporware -- in fact, it's nearly ready for distribution; here's what Diego has to say:
"I didn't want to release spaces until I could use it myself as a PIM, and it's basically there. What remains now before a first release is to do proper packaging, write installation instructions, etc."
It's written in Java (J2SE v1.4), although he's taken the time to give it a proper Windows GUI, which looks very nice so far. Apparently the performance is comparable with Outlook.
It will apparently be open source, under a license not unlike that of Movable Type -- free for personal or noncommercial use.
Looking forward to seeing it!
I checked out the home page and it looks great. From what Diego says, it's working and handing his 7000 emails already... 100% Pure Java, it's got a UI that looks like a native app. Add me to that list of people super-curious to check out the code! Too bad it's not going to be free as in beer... oh well.