This article is from the Paper Money Collecting FAQ, by Bruce Giese firstname.lastname@example.org with numerous contributions by others.
Here's a list of some websites...
A more up to date copy of this FAQ
American Numismatic Association (decent)
Krause is now online, although their page is wimpy and undeveloped.
A good site with images of USA and some World notes
History of money (good)
Irish banknotes and coins (ok)
Polish banknotes and coins (ok)
Slovakian banknotes and coins (ok)
Canadian Paper Money (ok)
World paper Money (ok)
Hamilton Coin Collectors page (has links to good pages)
U.S. Bureau of Engraving and Printing (wimpy)
Minneapolis Fed Site with image of new US $100 note
World Banknote Collectors Web Site
Smithsonian (good set of obsolete note images)
So far, in my humble opinion, nobody has created a truly Excellent
page for paper money. Take this as a challenge: First one who
gets their paper money web page designated "Cool Site of the Day"
(http://www.infi.net/cool.html) will be forever honored in the
Credits section of this FAQ.
2. Paper Money Collecting: "I FOUND A..." QUESTIONS
Paper money has both a market value and a personal value. These
two things are very different. There are big time dealers who
keep a note from circulation just because it has an interesting
serial number or for other sentimental reasons. Don't let "market"
forces blur the real meaning of being a collector.