A large set of resources for those who use Illustrator for wireframes. It includes 290 free vectors icons and 200 graphic styles, all free to use. This looks very useful indeed.
Design
User Interface Design Framework
Design Is History
"Part of the graduate thesis of designer Dominic Flask, this site was created as a teaching tool for young designers just beginning to explore graphic design and as a reference tool for all designers. It is supposed to provide brief overviews of a wide range of topics rather than an in-depth study of only a few. It is a constantly evolving, changing, expanding reference library. "
– from the site's about page.
13 Stripes and 51 Stars
An interactive exploration of the methods that could be used to add an extra star to the American flag.
Mixing Typefaces (PDF)
A cross-reference that indicates the degree of compatibility between fonts. This is ideal for selecting combination of fonts for a layout, though I wish it were newer and included a larger set (it was created in 1992).