Michael

In response to a question about flash choices for off camera lighting:

What is a cheap flash:
You have a few options in terms of what kind of flash to get, it all depends on what 'decently cheap' means to you. I like cheap as well, but if cheap means you need to replace it in a year or it may fail randomly, it may not be the best idea in the long run. If I buy the cheap option, I make sure I have a backup and plan that it will fail. Even doing that, the cheap option is often still much less then the expensive option.

What kind of flash:
Your options for a flash are the camera manufacture's current flash series, old model flashes, and off-brand flashes.

Current flashes:
In my experience, the latest-and-greatest from the camera's manufacture tends to work the best and have the most power. When I need something 100% reliable, I use my 580 EX II. Its more powerful then my other units, its much faster to recycle, and offers high speed sync and ETTL automatic metering when attached to the camera. The downside to it and the nikon equivalent is that they are expensive (~$400).

Used OEM flashes:
I have 6 older canon flashes which are my workhorses for most portraits. They have auto metering which doesn't work very well, you need to use them in manual mode. I use a 420 EZ (slow recycle, $45 ebay) and 430 EZ (faster then the 420 EZ, $55 ebay). Some people also use the 550 EZ, which is around $150 used and has high speed sync. The upsides to these are they are very cheap, the downsides are they are slow and only manual. You need to be careful with older flashes, some use high triggering voltages and can't be used on camera without frying the camera. I've never had a problem with the Canon series, but caveat emptier.

Aftermarket flashes:
The last option is the after-market flash. These include the vivitar series (285 hv is popular, manual only, $85 new), the lumopro 160 ($160, manual only, same power as 580 EX II) the quantum flashes (very expensive, manual only, extremely powerful), and lots of knock off products.

Power:
I think the vivitar series have adapters and the quantum ones may as well. Other then that, you can get external battery packs, but I haven't seen AC power. Get good batteries, I use Duracell's Pre-charged AA's ($10-12/pack), because they don't self discharge over time. Equivalent products are often labeled non-self discharge or eneloop. I have 10 sets now, and have yet to run out on a shoot. If you want something with wall power, I would just get a studio light, or alien bee's and a vagabond.

Triggering the flash:
Once you pick a flash, you need a way to trigger it. Some of the Nikon's have a slave built in, but its not reliable outside or in a room with other flashes. Non-built in options include a PC cord (reliable, but cumbersome; about $25 after you get hot shoe adapters), a flash sensor ($15-50, not too reliable), chinese triggers (I recommend the RF-602's, $30 for a set, semi-reliable; I have multiples so I have backups when they die), or pocket wizards (rock solid, but around $300 for a set).

Going to off-camera lighting is expensive, but will really make your images pop. Lots of the light modifiers can be bought on bay very cheaply or made in your garage. I would recommend not cutting too many corners on the light or the trigger, as you will likely come to rely on them. Good luck!

A great source for more info is David Hobby's blog at strobist.com. Check out the lighting 101 and 102 sections, as well as his posts on various flash options.