For the last two months, I've been using Chandler as my sole calendaring app on my Ubuntu laptop. Chandler is a free, open calendaring program with a lot of innovative rethinking of how to do groupware right — the web-based sharing technology is especially good. (I'm a very heavy calendar user and I really need industrial strength scheduling)
It's still very early beta, and there's a lot of polish missing from the current builds, but in the short time I've been using it, I've seen it make massive improvements. I'm really looking forward to future releases — give it a whirl, send 'em some feedback, or hack some code.
Chandler gives you the flexibility to collaborate with others on projects at a variety of different levels. Take full advantage of all the Chandler Desktop features by collaborating with other desktop users in your office to share read-only or writeable calendars, tasks, messages, notes and keep track of priorities. You can also manage a shared task list or calendar with others who prefer to use their web browser directly with Chandler Hub, they don't even have to have an account on the server to access the information you share with them.
Chandler Hub is a common connection to share your schedule and coordinate with other people. Chandler Hub supports you whether you're a committed everyday user or just 'dropping in' to leave a comment. Begin collaborating with other people today without all the commitments. Find flexibility in Chandler Hub–all the tools you'll need in work collaboration or to just simply keep yourself organized.