Taverna is a spin off from myGrid, and is available as an independant product under the LGPL. It can be obtained in binary form for Win32 and Linux (with a MacOSX release being part of the upcomming beta10 in the next week or so. Source is available through anonymous cvs or released archive.
Taverna can interact with arbitrary services, not restricted to web services. We write plugins that mediate the precise service interaction and semantics in each case, so we have one that talks to vanilla WSDL, one for Soaplab, Biomoby, local Java based plugins for shim services etc. Should there be any grid services that our user community wishes to make use of we will make appropriate plugins available.
Taverna supports its own XML language called XScufl that is roughly based on WSFL and created because Taverna authors felt that WSFL was far too fine grained for scientific applications. Scufl is abstract language and XScufl is XML representation of it. In past Taverna supported a subset of WSFL (besides XScufl) but this support is no longer maintained.(based on information provided by Tom Oinn).
Additional information: (X)Scufl documentation (via Wiki) Scufl language - conceptual overview and Taverna User Level Documentation (by tmo at ebi.ac.uk) Also Scufl models