We're in NJ and we've gotten flyers from both Constellation and Gateway Energy so I went to the BPU (state Bd of Public Utilities) to check out our options, now that NJ has been deregulated.
They give a list of each provider that is licensed to supply electricity/gas in your current provider's area, as well as their website so you can go to read the fine print. We're currently a JCP&L customer and ended up choosing Verde Energy as they offered the cheapest price per kWh and also support budget billing. They also offer a $50 credit - most of the alternative suppliers seem to be offering some sort of sign up bonus as they know that NJ customers have choices now.
Dig out your current electrical bill and compare the price per kWh that the competitors are offering - that's the best place to start. Some of the companies listed on the BPU site are actually more of a "green" provider and you'd end up paying more to go with them but you have the peace of mind knowing that you're using "cleaner" energy, if that appeals to you.
Start here - read through all the info and on the bottom you can click on your current supplier to get the list of alternate suppliers:
http://www.state.nj.us/bpu/commercial/shopping.html
They'll be listed with an "R/C/I" to designate which ones provide residential, commercial and/or indusdrial service. In our case, all we care about are the ones that supply residential service.
ETA - The same concept applies to natural gas as well. We have NJNG and when I compared all our gas options, none of them would give us much more than a 1% savings so I didn't bother switching gas providers. On the other hand, since the switch to Verde for the generation part of our electric, our electric bill went down just under 10% last month so that was worth it to me.
ETA2 - Our church recently switched over to Veridian and they had a little blurb in the church bulletin that if any parishoners switch over to Veridian, the church gets a $2 credit each month from each customer. Just wanted to toss that out as there are some affiliate offers apparently so keep your eyes open as you may be able to help out any causes that appeal to you... I considered it but I liked Verde's prices better.