I also do not keep track of the "value" of any of these stamps. If you have stamps like any of those depicted here and want to know what they're worth, I recommend a trip to your local library. Here in the U.S., most libraries have a copy of the Scott Catalogue in their reference section. The Scott Catalogue will give you a rough idea what it would cost you to purchase them from a dealer. (But don't expect to be able to sell your stamps to a dealer at the listed price.)
Outside the U.S. look for catalog(ue)s by Stanley Gibbons, Michel, and/or Yvertt & Tellier, to name a few.
U.S. Bicentenial Issues
Christopher Columbus Issues
U.S. Space Issues
South African Miniature Sheets
South African Special Miniature Sheets
South African Tourism Issue
Greenland American Series Reissue
Legends of the West