GeoServer is an Open Source server that supports WMS, WFS, and WFS-T standards. What makes it stand out particularly from most other Mapping Servers (both opensource and commercial) is that it supports Web Feature Server Transaction (WFS-T) specification and is the reference implementation for that specification. Most other servers have either no support for WFS or only support the regular WFS - read-only standard. This means that you can use it to update spatial data using XML/GML server calls as well as doing both attribute and spatial queries. Its WFS support in general is more robust than other mapping servers - e.g. compared to UMN Mapserver (which only support basic WFS) and ArcGIS which requires an additional extension and fairly non-existent support for WFS-T (*note ESRI has their own proprietary standard for updating spatial data between client and server).
Its WMS support is on par with other Mapping Servers. Geoserver can output data in numerous formats - JPEG, GIF, PDF, PNG, SVG, GML and KML/KMZ.
Geoserver is built on a Java J2EE Servlet platform. Common webservers used to drive it are Resin and Tomcat. It supports numerous datasources. Most popular ones used ESRI ArcSDE, Refractions PostGIS, ESRI Shape, Oracle Spatial, IBM DB2 Spatial Extender, Raster Data. There is preliminary support for MySQL and MapInfo.
If you need commercial support. Check out the GeoServer Commercial Support page.
MASS GIS uses GeoServer to publish some of their Oracle ArcSDE layers. Check out their example page on how to make WFS requests to their GeoServer. MASSGIS WFS Example Queries