What Time Tracking/Management Software Do You Use?

Specifically for billing clients and keeping track of how much time we are spending on a given project. I'm looking for something the whole team can collaborate on at one time. This software should be browser/cloud based. I don't mind paying for a great product. So BHW, what is your team using?

For those who are interested, after trying a lot of demos I finally settled on an open source program called collabtive. Just create a database for it in cpanel and drop it in a directory. It allows multiple admins, and multiple users to work on the same project. It has a built in chat function, you can export time tickets into pdf or excel, and it's pretty simple to use for the most part. This may not be the best solution if you have hundreds of clients but I can see us using this for 40+ clients.