Does anyone know of a good freeware or shareware contacts management system that one could download or use (maybe a web 2.0 tool) online?

I'm looking specifically for a contacts manager that has the following features:

1) Is cross-platform (windows, mac)
2) allows you to email contacts from within the software
3) uses tags or categories or some other way to identify groups of contacts

thanks for your help,
sage

Yes! My Robotics team uses PHPlist. It's really a great open source program. Check it out here: http://www.phplist.com/details

-Nathan