Dungeon Painter Online
By pyromancers, this site is a simple canvas with lots of features.
Select the tile you want to lay down and click.
Change grid type to hexes, dots, squares, or no grid. It also has a Stick to grid feature I found useful to toggle for fiddly tiles like doors.
http://pyromancers.com/dungeon-painter-online/

Cavern Generator
Not sure what to call this mapper, it has no name. But it's slick. And fast.
Unlike Dungeon Painter Online, where you create maps by laying down tiles, this generator does the map for you after you tweak the settings.
One click and you're done. I love it.
http://www.curufea.com/games/roguelike/map.php

Random Dungeon Generator
Next on my list is this procedural map generator by Inkwell Ideas.
It has just one setting to tweak and then you generate the map. Works great.
http://www.inkwellideas.com/roleplaying_tools/random_dungeon/

Dave's Mapper
This is my favourite mapper. It's mega-tile based, meaning each tile has about 10x10 squares worth of stuff in it, as opposed to a mapper like Dungeon Painter Online where you manage every 5 foot square.
Select the artists whose tiles you want to use, the map type, and then generate.
http://davesmapper.com/

Gozzys Random Maps
Create basic maps with this simple and clean procedural mapper.
http://www.gozzys.com/article.php?cm_id=11

The Isomage's House
If old school blue cavern maps are your thing, this mapper's for you.
Check out the other generators on this site while visiting too.
http://axiscity.hexamon.net/users/isomage/cavegen/