With this Java manipulative, you can represent place value and exchanges in base 2, base 5, or base 10.
http://nlvm.usu.edu/en/nav/frames_asid_209_g_1_t_1.html?open=activities&...

A Java implementation of Cuisinaire Rods. Very nicely done.
http://arcytech.org/java/integers/integers.html

A Java implementation of Base 10 Blocks. Very nicely done.
http://interactivemaths.net/beta/node/add/whiteboard-content