I found an old cell phone charger laying around and it gives out 5v 550ma. Would It work if I cut off the tip and just stuck the leads in a breadboard? Would I have to get a diode for it or not? Thanks for the help!
No it should work just as it is. Just be sure to measure and mark the proper polarity so you don't plug it into your breadboard with backwards polarity.
Lefty
Thanks for the prompt response! I will probably just solder the ends to a 2 pin header and use electrical tape to mark it.
It actually gives out 5V or it says so on the label ?. In my experience they are often two barely related pieces of information. They often don't bother smoothing the output of chargers too well since the phone battery does that for them when they are used for their intended purpose. I had one that said 3.7V 500ma, it worked nicely at about 8.5 volts after I stuck a 470uF electroltyic in it to tame the ripple.
The newer switching supplies for phones give off a very stable 5V. The same type of supply is used for iPhones/iPods, Blackberrys and most other cell phones. In fact any device that charges via USB connector uses that kind of power supply/charger. I have a few around my desk for the various gadgets I own and they make great Arduino power supplies.
You can tell the difference between the old transformer kind and new switchers by the weight. The switchers are very light and hollow feeling compared to a transformer of the same ratings.
@pluggy: didn't the voltage drop when applying a load? It's usual that chargers give out higher voltage without a load present and when reaching nominal current, voltage dropps.
About the diode, if it's on the arduino board, I'd say just leave it there. It's a nice protection against reverse polarity and it doesn't cost you anything. So in my oppinion, it would be the best not to solder your new power suply to arduino but buy a connector that fit's in arduino's power socket. Then you can use this power supply for multiple boards (at least that's what I did and it's quite handy, when having about 5 adruino boards :
)
Regards,
Peter
8.5 volt was when It was running a official ethernet shield which is a tad greedy. It was about 9.2 with just the arduino and 9.8 open circuit. It did 'pull' with load but perfectly OK to go through the onboard voltage regulator. It was an old charger with a transformer inside (that and a bridge rectifier and nothing else before I added the electrolytic).
I wouldn't make any assumptions about switch modes either, I have a few that originated with Netgear access points labelled 12v 1 amp (efficiency level IV 
) and light as a feather. They have the right plug and polarity for the arduino which is nice but they are the better part of 16 volts running a plain arduino. They pull a little with load but not as much as a linear. They are perfectly OK providing the 'other circuits' don't pull more than a few mA. Sticking your finger on the Arduino voltage regulator whilst its running is a good indicator, if it burns, you need less amps or volts or both....... 
I already gave it away (pics at http://www.aleyfun.com/2009/10/20/diy-5v-power-supply/). My first hot glue case and it turned out quite well to my standards. The hardest part was trying to solder the wires to a bit of 2pin header. I didn't have a third hand so I ended up using 6 pins of header before I got it right. It used to be a mini USB charger for a cell phone so I would assume a fairly stable voltage.