If you like artists like Norman Rockwell and Bernie Fuchs, or any other illustrator working in the middle of the 20th century, Today's Inspiration is the blog for you. Authored and maintained by cartoonist Leif Peng, the site pays tribute to American illustrators of the era, ranging from the Greats to the lesser known. He goes into great detail about the artists' careers and usually scans original ads and magazines from his own collection or the collections of an artists' family members.
Having never had proper Illustration History lessons, this site has been a great resource for me. Every day, I'm exposed to new paintings and artists, and am often treated with analysis of their work. It's like having a daily art history class delivered right in my Google Reader. Occasionally Mr. Peng brings in guest bloggers to discuss their areas of expertise. For example, I really enjoyed the week dedicated to Norman Rockwell (who wouldn't?), during which we get this close-up detail of one of his illustrations for Huck Finn. I recommend you check out the archives and check it regularly!