I draw all the maps for my 3.5 pirate campaign in Photoshop. I use some icons from the classic Hero Quest board game to indicate furniture and such. And the rest I just draw by hand. You can probably find plenty of pre-made maps right here on the forum. Several DM's have threads about their campaign on this forum, and such threads often contain lots of maps.
I've also used this random map generator. Be careful though as some of the monster combos make little sense. Like a 15x15 room with 8 ogres in it. Or a low level dungeon with a disintegrate trap on the door.