Dungeon Builder is an isometric map maker for just about every desktop platform. It is currently on Kickstarter for funding, but for now the duo of Aage Jørgensen & Kim Markfoged have put up a feature-limited demo of the program.
The links above go straight to the Kickstarter and any links below go to the demo download page.
While it is only a demo, you can still build some pretty usable dungeons, especially if you are just trying to make the rooms themselves. I built the bulk of the couple rooms below in a minute or two and then spent a while just screwing around to see how the editing went.
Dungeon Builder Demo, but you can easily take screen shots.
The Kickstarter is already funded and it'll cost about $45 to get the program through Kickstarter. That seems like a lot to me, but since I cannot draw isometric maps and wish I could, that price isn't terrible....
Obviously your mileage might vary, but for the price of free, I think the demo is well worth downloading.