MagazineArt.org is a trove of magazine covers and advertising illustration from the decades around the turn of the 20th Century.
Though a number of names of illustrators from that era have become familiar, many many more are still obscure, rarely featured or highlighted. The illustrations in the MagazineArt.org archives are heavy on the latter, light on the former.
If you dig, you can find some gems by artists like Maxfield Parrish (above, second down) Coles Phillips (4th down) and others.
The real draw here, though, is the unknown artists, both good and wonderfully cheesy. There is a search feature, but discoveries are best made by browsing.
You can start off in galleries sorted by topic (note the numbered links at bottom to subsequent pages) and drill down into individual titles.
Don’t be hasty to pass up categories that might not be of interest to you as a reader. Some of the best illustrations are in women’s magazines and weekly titles like Collier’s; some of the most fun are in the science magazines and pulps.