AUTHOR EDIT: It was requested that I explain exactly what "Pentesting" is. Pentesting is short for Penetration testing and while it can refer to the process of testing inked writing implements, I am referring to the process of testing a wireless access point, and underlying network, for security flaws.
While I am not a network professional, I can tell you that there are MANY tools used for doing this; some open and some proprietary. I also say that this is a Penetration testing platform because it refers to the lawful testing of your own access point against security flaws....Unless you work for a government agency, in which case you are the law and you can use it to steal network traffic from the bad guys. Now, let's get cracking....no pun intended.
Here's what you'll need.
Raspberry Pi Model B. $35 + shipping (maybe $42 max)
I recommend getting the Model B, because it has an Ethernet port, which is useful if you want to SSH tunnel into it for remote use.
USB to MicroUSB (May come with Battery Pack) Free if you have one or get it in with your battery pack. Otherwise I paid $15 for a Motorola charging cable before I got one with my battery.
Alpha AWUS 036H Wireless Adapter $40
I recommend the Alpha 036H because it is a very cost effective adapter and works well on the distribution you'll be using.
8GB Class 4 SD card (tested) $8 on sale
This is the SD card that I have and it works great. The image consists of a 4.1GB partition and 0.78 GB partition, so 8 GB's to be on the safe side.
You're going to image (dd) to this with pwnpi in this instructable: http://sourceforge.net/projects/pwnpi/
Gigaware Modular Powered USB hub With an extra male USB for powering the hub $8
I bought this from Radioshack and it works great, but you can use anyone as long as it take it's power from a USB cord not AC adapter.
5000 mAh power pack with 2 USB power ports. Powers is for about an hour. $14 + $6 shipping to New Jersey
I bought this one because it was cheap and works fine. http://www.amazon.com/gp/product/B004P8E612/ref=olp_product_details?ie=UTF8&me=&seller= . It was literally from China, the return adress was the Hong Shu Paint Building. You can also get them from Newtrent and other phone accessory places if you don't want to wait a week for it.
Mouse and Keyboard Free or cheap (depending on if you already own them)
Some little existing knowledge of Linux commands and working with wireless drivers. Knowledge is FREE!
Now that we have our parts and our thinking caps on, let's get to it!
Step 1: Imaging your SD card.
Download and unzip it with your program of choice.
Next, we must image our card with a program. I use ImageWriter from the Ubuntu Software Center.
If you're using linux, you can dd it to your card as well.
If you're using windows, you can use https://launchpad.net/win32-image-writer/
At this time, you may want to start charging your battery pack.