I found myself using Mappy last night just to test whether a tileset of mine would work seamlessly. It was handy because it allows me to zoom in and out, so I could check the pixels were lining up perfectly.
I also like that Mappy has the function to export a BMP file of your entire map. This is often gigantic of course, but very useful. We often use it to print out our levels on several sheets of A3 to plan things out further.
Of course, writing your own tile editor is always preferable.
![Smiley](https://forums.tigsource.com/Smileys/derek/smiley.gif)
Depends on your game really. I'm an artist more than a coder though, so I tend to avoid re-inventing the wheel whenever possible in order to get work done quickly.