In the course of a day I launch two browsers over and over again. I used to launch three, but I've killed Safari, for now. So I am fluidly moving between Google's Chrome and Firefox all the time. What I like better about one of them I don't like something else about the other. I can cache my WordPress site with Google Gears on Firefox but I can't on Google Chrome (figure that one out for me) and I am starting to get frustrated with the limits of each one.
But far and away my most sought after feature for guiding projects and teams who are in different locations is: An issue tracking system that allows the conversation to be threaded using traditional email. There is nothing worse than trying to manage deadlines or content assets via email. Especially when the volume, due to multiple projects, gets above 30 emails an hour.
It's an open world out there. Free tools to work on a closed system verses free tools to work on an open system. A system that is not OS dependent or processor dependent or language dependent.