- Google Apps (standard edition) - Free email and calendaring on your own domain name
- PivotalTracker - Free project management tool perfectly suited for managing development features for a small team
- Dropbox - Free "fileserver" for sharing any files across team. All team members get a "My Dropbox" folder in "My Documents" which is always up-to-date
- SliceHost - Incredibly cheap server-space -- lower-priced plans than Amazon for getting started -- $20/month with no setup cost for server that should handle most consumer internet sites until user traffic really takes off
- Viral Marketing Calculator - Amazing calculator for determining user growth
- Twitter - Every entrepreneur must be on Twitter to stay "plugged in" and to market his/her product
- VNC, Putty, Filezilla - Free tools for screen-sharing, SSH/telnet, and FTP respectively
Addendum: This post from Peldi Guilizzoni on the Balsamiq Company blog significantly extends the list above and provides very useful commentary. It's a must read for startup teams. Thanks to Jeremy Levine for pointing it out!