I currently use BaseCamp (www.basecamphq.com) and love it. They do offer a free version of their software for limited users and projects. I used ClientSpot (www.myclientspot.com) in the past and it too was a great program. I decided to switch because at that time the reliability of team/company messages wasn't great.

You may want to check out Google Apps, with their "sites" feature you can create an Intranet of sorts and create your PM system. Another option may be MS Sharepoint.

-Becki Noles