Google has a nice little MP3 player you can use on your site, as Amit Agarwal found out by digging through Gmail code. Just append the URL of an MP3 file to "http://mail.google.com/mail/html/audio.swf?audioUrl=".
This online player gives you streaming capabilities and a Google Video-like interface with a progress bar and sound adjustments. I guess this is a good option for making podcasts available to play in your browser.