Time tracking software and online services I’ve looked at.
Hamster: Local application (Linux/Gnome). Minimal Gnome dependencies, thankfully. Intuitive, well documented, and easy to use. Easy to synchronize your tracked hours across your computers by simply syncing ~/.local/share/hamster-applet/hamster.db. There is no file locking, so if you are syncing remember to close Hamster on one computer before you open it on another. My pick.
KTimeTracker: Local application (Linux/KDE). Open source and no cost. Not my cup of tea, but a robust and useful tool with good KDE integration.
Toggl: Online service. No cost for basic plan; paid plans add additional features. Attractive and easy to use. Their desktop application didn’t work well for me (Xfce on Linux) but they might work for you.
This list was last updated 17 November 2014.