Nick Denton of the Gawker has a post on the tools he finds useful in a start-up environment.  When we founded SelectMinds in 2000, most of these tools were not around.  We used AIM and Webex as tools, outsourced HR management and web design.  Most importantly, we went to a friend and had ourselves "incubated" – as in housed in the excess office space they had and hooked on to their LAN and phone system – in exchange for some equity.  This saved us tons in rent, telecom, IT support, and other overhead.  In addition, it provided us with a friendly, stimulating environment and bright people, instead of just the four of us (the founders).

I’d definitely consider a similar incubation model if I were starting a company, again.

As for additional tools, I’d list Bugzilla for bug tracking and Kwiki for knowledge management.