I've been posting a great deal about duration. Here is a list of four sites that are terrific in helping one determine copyright duration.
Laura N. Gasaway's famous chart, "When Works Pass Into the Public Domain,"
http://www.unc.edu/~unclng/public-d.htm - a classic, in any town.
Mary Minow's chart, http://www.librarylaw.com/DigitizationTable.htm - does it by year (super cool)
From Peter Hirtle's copyright duration chart, http://www.copyright.cornell.edu/training/Hirtle_Public_Domain.htm - does it by category and is more complex (including international) than Gasaway's. If you are unfamiliar with these kinds of charts, look at Gasaway's first, and then move to Hirtle's.
Copyright Term Cheat Sheet - Posted by Jason Schultz at Copyfight - July 16, 2004
http://www.copyright.cornell.edu/training/copyrightterm.pdf - This one is nice b/c it also includes int'l information.
For those new to copyright, remember. Again, anything in the public domain is free and clear to use WITHOUT ASKING PERMISSION. Anything still under copyright requires permission from the copyright holder (see posts about obtaining permissions and finding copyright holders), unless one is asserting fair use (a bit problematic, more posts to come on this).