Mapserver is an open source web based mapping platform.

Like most web  maping systems it allows you to connect to your data and other data out there on the web and display it in a manner you choose, then make it available to others as a web page, images or live feeds to get sucked into other web mapping systems. It uses common coding languages and components and can be quite fast in most implementations. It seems to be better suited to resource management or  government asset type of systems rather than demographics, campaign and common nonprofit data. The thematic mapping abilities are average.

Their website has a good set of walkthroughs to help you get started but you will need a development server to play around first and then a dedicated web server to install it into production over the web or in-house.

The model used for most implementations is something like this:

Some great examples of this technology can be see here:

• City of Bundaberg
• Gulf of Maine Habitat Monitoring