Recently at my $dayjob the remote workers were asked what collaboration tools we used to get our work done.  I thought this was a good topic and so I provided the following:

1. IRC - This is the bread-and-butter tool for communicating with people inside and outside of my team.  If I go more than an hour without looking at IRC there is no telling what I might have missed.
2. E-mail - Sometimes you need to send a lot of information to one or several people at a time.  I *only* use e-mail when I can't get someone up on IRC (they are out of the office, they work on the opposite side of the globe, or they don't use IRC) or have a large amount of information I need to convey.
3. git - There is nothing like collaborating like knowing everyone is on the same page as you are and you can track changes back.  I write a lot of documentation and it all lives in git.
4. svn - Like git, svn works well although we are trying to move the last bit of data that lives in svn into git

I think this is a good list but I also wonder what other people use for collaborating remotely.  What do you use?