Contacts
We've just open-sourced Contacts, our light addressbook that uses Evolution Data Server as a backend. Although it was designed for handheld devices, its very usable on desktops too. Chris Lord, who wrote it in the summer whilst interning at Opened Hand, has more details and screenshots.
For a small calendar application, we've already released Dates, get it via SVN from http://svn.o-hand.com/repos/dates/
Having stuggled with older versions of Microsoft WinCE it is also really nice to be able to test out Palmtop apps without needing to mess around with emulators and jump through various other hoops. (Xnest + Matchbox can be added in something closer to an emulator with far less work.)