Look at original StarCraft maps online?
Here are the official images from battle.net: http://classic.battle.net/scc/lp/sc.shtml
This includes all maps on the original game disc, and from Brood War. They are sorted by the number of players recommended for the map (click on the links at the top to see the maps).
The answers given only list maps made by Blizzard. For professional maps, you should look here: http://www.teamliquid.net/tlpd/korean/maps/
While Quantumgeek's answer is a good one, if you want, you can load up the maps in the map editor, and you can see all the detail you want too, but that's not online.