Alien Bee 1600W
If you ever do any kind of studio type shooting, the Alien Bee is absolutely one of the more affordable units to start with. They come in different max output powers with the B1600 being the most powerful of the “B” line. I rarely use full power, but have on occasion when shooting into very strong back light (such as a sunset). There is also a very portable battery pack you can purchase (See below) for on location shooting, and each of the units has a “slave feature” meaning it will trigger when the main flash fires.
My only criticism on the Alien Bees is that Ive had the plastic switch break on me, but they will repair it and their customer service is wonderful.
For a long time I only had 2 of these units, but when I got into High Key Shooting I got a third. At $360 it is a steal of a deal, and the B400 and B8 models are even more affordable.
Highly Recommended
Purchase Alien Bee B1600
Vagabond Mini Battery Pack
Really intended for use with the Alien Bee units. Its almost small enough to fit into a camera bag. Much improved over the older Vagabond (which was not really portable). The thing I love about the mini is how small and light it is. If we have an on location shoot, one of these has always sufficed, even though it is rated for about 260 shots. I have never run out. Comes with a shoulder strap and charge cord.
If you have an Alien Bee, the vagabond mini will allow you to take it as far away from an outlet as you can carry it.
Purchase Vagabond Mini - $240
Pocket Wizard Plus II Transmitter Receivers
A critical device for any serious studio shooting, remote triggering, syncing of different types of light systems, etc. Pocket Wizards are the utility device of nearly all serious pros out there who need a way to transmit and then receive a radio signal to different types of equipment. Because they are radio based, they work through glass and walls. With an accessory cable, you can use 2 to create a long distance remote release. (Ive personally used it over 200 feet).
Highly Recommended for Studio Shooting. 
Purchase Pocket Wizard II Transmitter Receivers
