ESRI's ArcExplorer is a lightweight, no-cost GIS data viewer. Note that the .NET version of ArcExplorer is significantly different from the Java edition.
ArcGIS Server is ESRI's network- and webservice-centric GIS application suite. Among other, ArcGIS Server offers WMS, WFS, WFS-T, WCS, and KML service capability as well as web application SDKs for JavaScript, Flex, Silverlight, .Net, and Java.
GeoServer is an open source software server written in Java
that allows users to share and edit geospatial data. Designed for
interoperability, it publishes data from any major spatial data source
using open standards.
Being a community-driven project, GeoServer is developed, tested,
and supported by a diverse group of individuals and organizations from
around the world.
