The very nice Timetracker application now runs at some 90% under Linux.
The software is a dotNet application that runs in Mono. Under ubuntu (Jaunty 9.04) you need the core that is probably already installed and:
sudo apt-get install libmono-microsoft-visualbasic8.0-cil libmono-winforms2.0-cil
The only annoyance right now is that the software is distributed as a .exe file that cannot be run in Mono. You have to either install the software in a Windows machine and then copy the installation directory to your Linux machine, or use Wine to run the Installer.