Other posters have given several solutions, just collecting and adding my voice to a few of them:
in a pinch:
Google docs: lightweight and simple, with limited functionality and a light learning curve
Sharepoint: simple to use, full of hassle to administer, limited functionality, gets expensive
mediawiki: like sharepoint without the licensing problems, but gets limiting beyond simple document collections
More serious solutions:
Alfresco: serious document/object management and workflow, free version to start/pay for support if you like it (spinoff of Documentum)
Documentum: elder god #1 of doc management, excellent repository, workflow, project management functions. rather expensive
Opentext Livelink: elder god #2 of doc management, excellent repository, project management, nice Visio-like workflow development that makes sharepoint devs cry, also rather expensive.
Google docs if you need a fix today, Alfresco if you have a month or two to fix the problem and want it to stay fixed.