Pinyin tones pronunciation cheat sheet
Here are my two favorite browser-based pinyin cheat-sheets:

https://chinese.yabla.com/chinese-pinyin-chart.php
http://www.quickmandarin.com/chinesepinyintable/pinyintable_vertical.php

Whereas the site linked to in the currently top-ranked answer (lost-theory.org - a nice, new find for me, BTW) uses big, lossless .wav files (which also unfortunately seem to include a great deal of background noise), the sites I link to above use compressed mp3s, so I think you'll have a vastly easier time using them on a slow connection. Enjoy!