IRS Donation Guidelines (tax year 2007)

Federal law permits donors to claim tax deductions for many financial contributions and for donated clothing and household goods in good, used condition or better. If you’re unsure whether your item qualifies for a tax deduction, then consider this: if you would give it to a relative or friend, then the item is most likely in good condition and is appropriate to donate.
The U.S. Internal Revenue Service requires donors to value their items.

Government rules:

http://www.irs.gov/pub/irs-pdf/p561.pdf 

http://www.irs.gov/pub/irs-pdf/p526.pdf 

http://www.irs.gov/pub/irs-pdf/i8283.pdf 

Helpful evaluation site from Goodwill and Salvation Army: 

http://www.goodwill.org/c/document_library/get_file?folderId=102123&name=DLFE-2302.pdf 

http://www.salvationarmyusa.org/usn/www_usn.nsf/vw-search/85256DDC007274DF80256B80003D22FC?opendocument