Ask HN: hosted time tracking tools for distributed teams?

I have a distributed team of a couple of software developers who work on hourly basis. I am looking for recommendations of online tools that let them submit the hours they work in a way that is minimally intrusive and that help me keep track of the hours. I'll be using this for billing purposes too. I'm currently using assembla.com for bugs/svn, they have some time-tracking feature but it's not very easy to get reports on hours worked. I want to keep using assembla for svn/bugs and I am looking for an additional tool.