FreeTime is a free, cross-platform, GPLed replacement for the terrible-and-windows-only TimeWatch PersonalTime client my employer decided to use. Implemented using my OOweb framework to provide a web interface.
This replacement client requires that your CentralTime server has your user set to use file shares for exchanging data, rather than via email. The file share will then need to be accessible via your filesystem (ie. Mounted via CIFS/SMB under linux or have a mapped drive letter under windows).
Requires any free Java virtual machine.