If you don't have one, you should get one.  Only 139.99 shipped (or 129.99 if you use Google Check Out).

From Amazon

From Buy.com

You may also want to get one of these:

1 gig
2 gig

Not essential, but it'll make things run faster (see the Starting out guide) and give you more space for videos/mp3s/applications/maps/pdfs/e-books/etc.

You may also want a GPS unit - some cheap ones here.

Check out a few of my posts about using the 770:

Making the 770 do cool(er) stuff
Starting out with the 770

Pros:
+ Can connect to internet using wifi or bluetooth enabled tethered phone
+ Runs linux
+ SSH/SSH Server
+ VNC Viewer to check in on other computers
+ Large open source community working on applications
+ Real Opera browser
+ Opens PDFS
+ Can connect to a bluetooth gps unit and do directions (see the starting out guide)
+ Cheap!

Cons:
+ Slightly slow (I haven't found this to be the case at all)
+ You have to be somewhat technical to do some of the more advanced things with it
+ Can't access a few heavy java-script sites like GCal yet.  Opera support is coming.
+ Can't easily view YouTube videos because of the version of flash player.  This might be coming - I don't see why it wouldn't.