WebCGM
WebCGM is not an XML application and so I won't go into much detail about it here, but it does at least deserve a mention.
CGM (Computer Graphics Metafile) has been an ISO standard since 1987, and it's been a recognized Internet format (MIME type) since 1995. However, although it is the preferred format for technical documentation it hasn't made much of an impact on the Web (there is a browser plug-in that allows you to view CGM data). There isn't that much mass-market software that fully supports it either. Most graphics packages allow you to view, and possibly write, CGM raster data, but there are very few packages that can deal with CGM vector data. Nevertheless, Boeing (one of the major users) and some of the leading software companies in this area (including Intercap, Inso, and ITEDO) have proposed a profile of CGM called WebCGM that introduces metadata that can be used for hyperlinking, document navigation, and searching based on picture content.