Fosdem
in Brussels (Belgium) is one of the largest, probably best OpenSource conferences in Europe. A link to the 2005 event:
TheoVerelst happens to present at the Tcl Developers room if all goes well: http://www.fosdem.org/2005/index/dev_room_tcl . I've made a page about the 2004 conference: http://82.168.209.239/Fosdem/