I just received my brand new WiFi Pineapple Mark IV, also known as the "Jasager" from Hak5.  I thought that since I'm starting a blog I might as well do some product reviews as I get new toys.  The Mark IV is an Alfa AP121U 802.11 b/g/n wireless router running OpenWRT and loaded with Karma, URLSnarf, and DNSSpoof just to name a few.  The AP121U has one USB port, one RJ-45 LAN port, one RJ-45 WAN port, one antenna connector, and a WPS button.  It is built on the 400 MHz Atheros AR9331 SoC. It ships with a 5dBi detachable antenna, a retractable ethernet cable, and a power supply.  I ordered the USB power cable ($5.99) from Hak5 so I could power the router from my laptop instead of having to plug it into the wall. 
The primary function of the device is to take advantage of the feature built into most systems that allows ease of access when connecting to known WiFi networks.  When you are at home you can just open your laptop up and voila you are on your network.  In orderfor your computer to know that your access point is close to you and not your work network it has to send out probe requests.  Probe requests help your laptop know what to connect to.  The Mark IV listens specifically for any systems sending probe requests.  When it sees one it will tell the system sending the probe that the Mark IV is the AP they are looking for.  This is where it gets it's From a penetration testing point of view you could potentially walk into the company you are (legally approved) to test and see how many laptops and cell phones automatically connect to you.  You can even blacklist and whitelist AP SSID's.  This allows you to get more granular in your test which will help you not affect users who are not part of the organization you are testing.  Potentially, you could leave the router plugged in to the network you are trying to penetrate then connect to it from the parking lot over WiFi to conduct scans of the internal network remotely. You could even use a 3G USB card and set up a VPN so you can do your scanning from home.  It also has an autostart feature that allows you to just turn the router on and you are up and running.  Another great use of the pineapple would be to set it up in your own office to catch devices before bad guys do.  You could use DNSSpoof to do an awareness training of sorts.  Think about redirecting every user to a webpage that describes what just happened to them.