regsvr32 "C:\Program Files\Microsoft BizTalk Server 2006\Developer Tools\BtsAsmExt.dll"
(where C:\Program files\Microsoft BizTalk Server 2006 is my BizTalk install location) and then take a look at Windows Explorer. There is an extra menu item called "BizTalk Server Assemblies."
Opening this menu item shows all deployed BizTalk assemblies. Opening any of these assemblies will show you all the BizTalk artifacts that are part of the assembly. You can double click on any of these (a schema, or an orchestration, for instance) to get an XML representation of the artifact.
Clicking View > Explorer Bar > BizTalk Server Search will open up a pretty good search tool for searching across all artifacts. The search results will also tell you what references the items displayed.