I have serveral different after market chargers not provided by samsung that are working fine. In theory all USB chargers should work but some, made for lower powered devices, aren't effective. The original USB standard did not anticipate devices drawing so much power as some of the phones. I have a car charger with two ports, each labeled 1 amp, that charges my N2 and another current Windows phone effectively. My after market chargers at home and work also seem to be effective. I do notice that sometimes charging is very slow for no apparent reason even with the Samsung charger. It's a situation that comes and goes.
Don't shop chargers strictly on price. Read some reviews on amazon and see what users say about particular devices.
Therer are now AC wall outlets available with built in USB ports. If the prices aren't ridiculous I'm tempted to get a couple and see how they do.