The Mobile9 Gallery has a nice collection of themes mainly for the newer Nokia S60 3rd Edition devices like 3250, N91, N80, N93, etc. There’s some nice themes like Symbian Vista, Mandrake Linux, cars, design, etc.

Link-http://gallery.mobile9.com/c/series-60-3rd-ed-themes/1/

Updates:
Mobile-reviews.de
babinokia.com
NokiaDownloads
Symbian Themes
MMCForums