I'm not a stamp collector by avocation. I just accumulate them. I think I still have some 15 centers in a drawer somewhere. At least, I think they are. At some point or another, the USPS decided it was some great idea to stop putting the rates on the stamps. It's a conspiracy, I tell you. It's so we buy new ones because we don't know how much the old ones are...

Well, http://alphabetilately.com/G2.html to the rescue. It's a listing of stamps by year, with pictures (never mind that recent ones are copyright by the USPS) and their value.

Of course, I don't have the 1, 3, 5 etc. penny stamps to make up the difference, so I resort to sticking two of the older ones on, just in case.