WorldPainter is an interactive map generator for Minecraft. It allows you to "paint" landscapes using similar tools as a regular paint program. Sculpt and mould the terrain, paint materials, trees, snow and ice, etc. onto it, and much more.
WorldPainter is implemented in Java, which you should already have if you're playing Minecraft. If not, install Java first from here.
Download the program here:
See this page for more information. VirusTotal results for these download links can be found here.
32-bit version: download
(works on 32 and 64-bit Windows, but not on 64-bit Java)
64-bit version: download
(only works on 64-bit Windows and Java)
Mac OS X: download
Installer: download
(all versions of Linux; can be installed as root or as a regular user)
RPM package: download
(Red Hat, Fedora, etc.)
DEB package: download
(Debian, Ubuntu, etc.)
If you have trouble installing the program using one of the installers above, you can download installerless archives here: 32-bit Windows, 64-bit Windows, Mac OS X and UNIX/Linux. Note that these are not recommended and unsupported!
These installers were created with 
.
The source code for WorldPainter is here.
There is not much documentation yet, but the program should be pretty self-explanatory. To play the map, choose Export in the File menu. Just experiment and try everything out. Don't forget to check the menus, and to see what happens if you right-click instead of left-clicking.
Having said that, the documentation, such as it is (including a FAQ), can be found here.
For support, go to the official thread on the Minecraft forums.
WorldPainter was created by pepsoft.org, a one man Java software development company from the Netherlands. It is not affiliated in any way with Mojang and has no official status. WorldPainter includes the work of many others - the credits page lists them.