Also some good companies to search for (Not in any particular order):
Rainbow
Infinity
Polk (If you got the cash, Polk Momo's are GREAT speakers)
JL Audio of course
Rockford Fosgate is very expensive and the sound quality is pretty good, but if your expecting something AMAZING from the price, you're not going to get it...
Boston Acoustics
I would recommend you do NOT buy sony, or most of the type of speakers that they sell at places like best buy or circuit city.. The best place to go for speakers would be an actual car audio shop..