Replicon Web Timesheet (Score 2, Informative)

We use Replicon Web Timesheet. It's a web-based solution, the website is usable by Firefox, IE, and Opera. The server runs on the Windows platorm, and uses a Microsoft or Oracle database. Of course, if you're not a Windows shop, this probably isn't the answer for you.

It works great for us. We've been using it for 7 years now, with 40-ish users. No problems, it's a great product. Entering time is easy, the reports are powerful, and it can integrate with other software. We integrated it very easily with our in-house account-management system.

If you don't want to run the software yourself, I see they have a hosted offering.