Karel the Robot

Karel++
Object-oriented version of Karel.
http://csis.pace.edu/~bergin/karel.html

Karel Home Page
Tutorials, downloads, and information.
http://www.mtsu.edu/~untch/karel/index.html

Karel J. Robot
Java version of Karel.
http://csis.pace.edu/~bergin/KarelJava2ed/Karel++JavaEdition.html

Marvin
A Java-based RoBOTL interpreter.
http://www.kluge.net/mqp/

Sourceforge: Karel The Robot
Karel for Unix.
http://karel.sourceforge.net/

xKarel
Open source port for Windows and Linux.
http://xkarel.sourceforge.net