Here is a list of some gear you can use to create a studio in the field. I have many of these items and use them almost every day. Let me know if you have any questions on this list or if you have any suggestions for additions.
Nikon SB-900 Speedlight - if you have the budget, this is the best for Nikon shooters
Nikon SB-700 Speedlight - great hot shoe flash
Nikon SB-600 Speedlight - discontinued but you can still find them on the market (for now)
There are many other hot shoe flash options out there. Depending on how you trigger the flash unit, you can use off-brand speedlights (i.e. Vivitar, Lumopro, etc.)
Wescott Collapsible Umbrella - shoot through & bounce
Collapsible Softbox - Lastolite Ezybox 24x24 Kit with Adapter to mount on light stand
Large Umbrella - Photoflex 60" shoot through and bounce umbrella
32" Collabsible Reflector - White, black, gold, silver and translucent
Stands, Adapters, Grip
Impact Umbrella Adapter - Use this to attach an umbrella to the a light stand. This one can be flipped over to use for adding other accessories. Others have a fixed cold shoe that can only be mounted in one vertical direction.
Compact Light Stand - lightweight, 7' stand.
There are many other options for light stands (i.e. Manfrotto's compact stand)
There are now many options for triggering your off-camera flash. There are radio triggers like the PocketWizard Plus II and other off-brand radio triggers. You can use the infrared trigger built into your camera's pop up flash, if available. You can find that functionality in the Nikon D80, D90, D7000, D300, D700 and some others. Canon also supports this function in its newer bodies such as the 7D and the 60D. You can also use a cable from the camera to the flash.
Check out the new PocketWizard Plus III transceiver.
Adobe Lightroom 4 - great for shooting tethered directly to the computer and for robust image editing and organizing.
Apple Keynote - presentation software for the Mac. Get if from the Mac App Store for all of your Macs.
The Nikon D800 for professionals (probably overkill if you're not a working pro, unless you have the extra money)
Nikon 24-70mm f/2.8 - this older lens is still a workhorse among pros. It provides amazing clarity, contrast and richness in images. It's a bit heavy but worth every ounce (IMO). Tamron recently released their version, which includes vibration control (for Nikon, for Canon). I haven't used it but it might be worth checking out. From what I hear, Canon's version of this lens is also good but not quite up to the same level. I also hear that the Canon 24-105mm f/4 is a very sharp lens.
Nikon 70-200mm f/2.8 - the latest model of this outstanding lens includes Nikon's VRII for improved vibration reduction. I have used the older model for some time and love the results. This is a full frame lens so it works on full frame bodies and is cropped for the cropped sensor cameras (anything but the D3/D4/D700/D800). This is not a cheap lens but provides great results! Canon also makes an amazing lens in this category. Check it out: Canon 70-200mm f/2.8 (there is also a non stabilized lens version too) or 70-200 f/4.
Here's a great tool for simulating the focal lenths of different zoom lenses on full frame and crop sensor camera bodies. Nikon Lens Simulator Tool
Wireless Network - I'm using a portable travel wireless router for those times when no Wi-fi is available and I want to transmit pictures from the camera to my iPad. I'm trying out the Sapido MB-1132 3G/4G Mobile Hotspot w/ High Capacity Built-in Rechargeable Batteries, Power Saving and Portable, 300 Mbps Wi-Fi 802.11n Broadband Router w/ USB 2.0 Port and Dual Active Firewalls, Compatible w/ iPhones, Androids, Blackberry Phones. It's battery operated so you don't need a power outlet.
Memory - I'm using SanDisk memory cards (there are some current promotions over at B&H). I find them reliable. When shooting on the job, you need to feel comfortable that your memory cards will work every time. Can't take chances on losing shots.