Webster's on-line dictionary. Has audio facility. Hear words spoken in AmE

http://www.m-w.com/

BBC learn English site. Has audio in BrE
http://www.bbc.co.uk/worldservice/learningenglish/index.shtml

Randall's basic self-study guide. useful words and phrases in AmE accent
http://www.esl-lab.com/guide.htm

AmE pronunciation (you need Real player for this one)
http://evaeaston.com/pr/a-pattern.html

This site says they offer free software
http://www.free-english.com/english/Home.aspx

Users should note that http://www.free-english.com/english/Home.aspx is not, strictly speaking, free. You will be required to register and to give personal details such as your address, telephone number, date of birth etc. etc. You will also be required to pay 'shipping fees' on any items you add to your 'shopping cart'. (Thanks to Mike in Japan for this info)

Another talking dictionary
http://www.answers.com/main/ntquery?s=dictionary#Dictionary

This is a great site for phonectics: animated diagrams to show production of sound, video and audio of certain words.
http://www.uiowa.edu/~acadtech/phonetics/about.html

http://librivox.org/ offers free audio books, and you can be a volunteer to record and add new tiltles to the database.