That's an easy answer: my electronic keypad of course! A very valuable device to open electronic keyboxes to obtain entry into properties. The second item would be my Tom Tom for ease of navigation around the streets in towns I'm not as familiar with.
Prudential Connecticut Realty
You can also use it as an open house register: http://openhomepro.com/
I am just learning to use an Ipad to load info via Drop box. check it out...
I like my gps to find properties, but nothing beats doing a little home work and finding the address on a map or even driving by first. my GPS ocassinally takes a nap and leads me down a dead end road.
Wireless card to plug into a computer if not in an area where I can access free wifi
Digital camera with wide-angle lens capability
If your GPS is taking you in the wrong direction, you might want to update your service. I have heard of that happening. Usually an updated map application will solve that issue.
My Blackberry will not be getting renewed, as I have seen the future and it is the Windows Phone. But I don't use a laptop for much, and I could get along just fine with just a phone, keypad, computer, and printer.
But, my "Must have device" is my Blue tooth ear piece! For Safety and convenience, I Love it!