Talkingstreet
Talking Street offers a convenient way to explore a destination and discover where you are. Just choose a stop, call the phone number for the tour, and enjoy an audio segment about the place where you're standing. You can visit the stops in any order, and at your own pace--once purchased, you can access the tour for up to seven days! From the Tea Party to Fenway Park to the Molasses Flood of 1919, Boston is full of great stories. Hear them come to life around you—on your schedule and at your own pace. Steven Tyler's lively narration, you'll hear expert interviews, eyewitness accounts, local voices, original music and more.
Cost - $5.95

Tourcaster
Download your own personal tour guide and take yourself on an informative guided tour of historic sites around the world. Look as you listen. Walk freely while on vacation and learn at your own pace. Each downloaded tour comes with multiple mp3 files and a map for each city or tourist destination. Maps are numbered with stopping points that correspond to each audio track.
Free to download - however you must register.

Boston HarborWalk Free Audio Tour
Download a free MP3 audio tour that you can play on your iPOD or other MP3 player. The tour covers the waterfront area starting at Columbus park and continues through Long Wharf, Central Wharf, India Wharf and past Rows Wharf down to the Childrens Museum. Files are compressed in a .zip file and roughly 1.3 MB. Offered free of charge by the The Boston Harbor Association. This site has a lot of good background detail on the Harbor Area - Interactive Maps, photos, and information.
Free to download

Ben’s Boston Freedom Trail Walking Guide
Get an hour of commentary on your MP3 player as you walk along the freedom trail.
Free to download