I have a HTC Hero and when I got it I spent quite a long time trawling the App Marketplace looking for cool apps. I found many that are awesome but I thought I'd mention this one as it's relevant to us all.
It's called gStrings. [url="http://www.cohortor.org/gstrings"]http://www.cohortor.org/gstrings[/url]
A full chromatic tuner that just uses the built in mic on the phone. Probably would work better on Guitars but I've not had any problems using it on my bass.
If you've got an Android phone, I highly recommend it. What's more, it's free as well!
Search for it on the marketplace, you won't be dissapointed...