Here's what I have so far that could be useful during an onsite pentest:

Pwn Plug
WAP
Hardware key loggers
Infected CDROM labeled something enticing like "layoffs"
Infected USB drive
Teensy USB HID Attack Vector from metasploit
Pen that records audio
Shredder that scans with OCR before shredding (heard about it, but haven't seen one)
Camera to take pictures of sticky notes, documents, etc.

Anything else you guys use?

@ChrisG That's a good one, thanks!

I forgot about the Ethernet cable, thanks.

These aren't really technology, but useful:

Fake business cards
Briefcase to get stuff in or out
Clipboard to look like you're working on something

That isn't in your laptop bag??   If the scope for physical security is included, sure why not.

Not all the stuff I have in my backpack, but here's a few suggestions.

1. HDD - USB adapter. Makes it easy to just pull a hard drive and copy stuff in a hurry. They're cheap:

http://www.newegg.com/Product/Product.aspx?Item=N82E16812156102

Or you can get an expensive one that dupes drives in a matter of minutes like this one:

http://www.logicube.com/products/hd_duplication/echo-plus.asp

Logicube even has devices for hard drive as well as mobile devices that are specifically made for forensics investigations. Kewl stuff.

2. I also like ChrisG's suggestion. I personally like Offline NT Password & Registry Editor:

http://www.pogostick.net/~pnh/ntpasswd/

Don

I have never been on any professional pen test 
but let me add to the list..

Network hub or switch
Live CD's
Blank CD/DVD's
Power strip

I once had a faculty who was a professional Incident response handler, I had a chance to peep into his 'Jump bag' and they carry so much, things like screw drivers, spare screws and variety of hardware interfaces to connect to devices on site . And he used to say never steal from your own jump bag