A couple of websites dedicated to Inkscape effects:
Official tutorials: Inkscape.org
Guide to a Vector Drawing Program e-book: Tavmjong.free.fr
35 Tutorials to create amazing Vector Graphics: Speckyboy.com
Inkscape tutorials web blog: Inkscapetutorials.wordpress.com
All these sites provide quite a load of learning material. Oh, and by the way, if you didn't have a chance to get to know Inkscape, its website is Inkscape.org it's completely free. ;-)
