Not sure what I like yet but I wanted to make a few notes.
12 & 13 bear a strong resemblance to an American telephone company.
http://wigab.com/images/site/2007-01/blog/44-Cingular_Dropped_Out.jpg
7 looks like barbed wire which isn't very inviting. I think only a slight change would
be necessary to stop that.
15 uses a hyphen which is not in the url, however I think the egg shaped background is
symbolic of the extendability of XWiki.
14 has an X which looks like it could be used in other places as an icon.
11 I like the hummingbird but the font seems mismatched.
Caleb