Got my T-Mobile DASH for about 3 months now. Beside all the great things, I was be able to manage to get some free Maps and GPS application working.
First, I got a HOLUX GPSlim240 BlueTooth from eBay for about $65.
Next, I used Map4PDA with Yahoo Map provider. For some reasons, Google maps doesn't show up correctly on my device. Today, I try to use GMM (http://www.google.com/gmm) and it doesn't recognize the HOLUX.
Here is the settings I found from xda-developers: http://forum.xda-developers.com/showthread.php?t=293018
Recap:
1. Assume that your GPS is connected on COMM port 6: COM6
2. Edit registry to enable it:
KEY: [HKEY_LOCAL_MACHINE\System\CurrentControlSet\GPS Intermediate Driver]
Add VALUE: "IsEnabled", DWORD set to 1

KEY: [HKEY_LOCAL_MACHINE\System\CurrentControlSet\GPS Intermediate Driver\Drivers]
Add VALUE: "CurrentDriver" STRING set to "BT GPS"
Add KEY: BT GPS

KEY: [HKEY_LOCAL_MACHINE\System\CurrentControlSet\GPS Intermediate Driver\Drivers\BT GPS]
Add VALUE: "FriendlyName" STRING set to "BT GPS"
Add VALUE: "InterfaceType" STRING set to "COMM"
Add VALUE: "CommPort" STRING set to "COM6:" (remember to include ":" too)

Restart your phone and it should work. Thanks pchronos for the tips!

One other option is to use Windows LiveSearch: http://mobile.search.live.com/client/download.aspx