The WMS Javascript Library provides an API to allow the creation of dynamic maps including simple zoom functionality, clickable googlemap-like overlays, and GetFeatureInfo queries by clicking the map (coming soon).
OpenLayers is a pure JavaScript library for displaying map data in most modern web browsers, with no server-side dependencies. OpenLayers implements a (still-developing) JavaScript API for building rich web-based geographic applications, similar to the Google Maps and MSN Virtual Earth APIs, with one important difference — OpenLayers is Free Software, developed for and by the Open Source software community.