Are you referring to the TopCoder Arena java applet? It still exists here: http://community.topcoder.com/contest/arena/ContestAppletPro... . They've redesigned the site recently but to access it you can just visit topcoder.com/tc and click Challenges -> topcoder Arena.

The browsable problem archive is also in the same location: http://community.topcoder.com/tc?module=ProblemArchive.