OK since this is the reprap forum I assume most people are either buying or scavenging parts for their build.

It is fair to make some assumptions that most users here have some electrical knowledge and know how to jump an ATX psu.

So lets discuss the different power supply types.

We have the cheapest almost free ATX. This type of PSU has allot of rails and it is not difficult to obtain for 30-40bux shipped to your door. However there are pro's and cons associated with this type of PSU.
ATX.jpg

The mod to turn it on basically green wire into black wire (Ground)
ATX-Mod.jpg
Pros
- Readily available.
- Can be quiet
- Cheap $30-$40
- Standard form factor making them easy to replace.
Cons
- Unstable 12V due to unloaded 5v & 3.3V rail
- Expense added to add load on above rails to make it perform properly (this adds heat as well if using a resistor bank.)

XBOX 360 PSU
XBOX.jpg
The MOD
XBOX-Mod.JPG

Pros
- Quite cheap 20$ or so
- Standard form factor.
- Silent
Cons
- Quite weak, 203W is the most powerful one
- Will be running at over 70% load if you have a heated bed.

Chinese 12V PSU.
Chinese PSU.JPG

Pros
- Inexpensive
- Stable 12v rail
- Fairly compact.
- Almost if not silent
Cons
- Have to be wired for high voltage 110v US 220v rest of the world.
- May have noise in the power (High frequency whine is not unheard of) or ripple in current.
- May have false advertising (Advertises more power than it can deliver)

See on next post for server PSU.