Below are several references used by Numismatists.
CATALOGUES
1. Krause Publications 2010 Standard Catalog of World Coins (1901-2000) [link]
2. Krause Publications 2010 Standard Catalog of World Coins (2001-2010) [link]
3. The Banknote Book [link]
4. Colnect Online Banknote and Coin Catalog [link]
* catalogues might take quite a long time to fully load depending on your internet speed and server traffic. please be patience.
WEBSITES
1. Don's World Coin Gallery [link]
2. Don's and Vic's World Banknote Gallery [link]