For OSX users:
http://www.malus.ca/EN/index.shtml
Sambucus is a great program!
From the site:
Sambucus is an intuitive time-tracking application aimed at freelancers, small businesses or anyone else who has a need to track his/her time and then report about it. Through a simple yet flexible interface, Sambucus let's you manage the time spent on different projects. You are constantly reminded that the clock is running through a non-obtrusive timer window (in both the classic and Mac OS X versions) and/or the application's dock icon (Mac OS X verion only). When billing-time comes, you can either print one of the supplied generic reports or bills or design your own through the flexible XML-like reporting language that Sambucus uses.
The base unit in Sambucus is the project: a project can be a real-world project or simply a client file. Each project can then be sub-divided into tasks (or client contracts if you use the project as a client file); the task basically stores the collection of time stamps (each working period spent on the task) that cumulates over time to be billed later to the client.
Add to all this the multiple "Undo" ability, the auto-backup feature, our commitment to make the application even more practical and useful without sacrifying it's ease-of-use and you've got a very nice time-tracking application for very little money... A winning combination, in our mind