XSP is eXtensible Server Pages - another idea from Apache. It is based on ideas from JSP and XML. It allows dynamic pages to be written as XML documents with embedded logic (in whatever language). The resultant document can be passed through XSLT and/or FOP if you want. As with FOP, there is currently only a Java implementation at apache.org. I (Neil Madden) am seriously considering a Tcl version, as this should be quite simple to do in pure Tcl.
AK: See also Template and Macro processing in general.