Calculate the last digits of Graham's Number
dc - 21 chars

[3z202&gt;xO200^|]dsxxrp


This takes about a minute on my computer, and would take lots longer for values larger than 200. It doesn't output leading zeroes.

Here's a slightly longer but faster version (26 chars):

[3rAz^|dz205&gt;x]dsxxAz6-^%p
3[3rAz^|dz202&gt;x]dsxxAz3-^%p # or an extra character for a few less iterations