[youtube=http://www.youtube.com/watch?v=m8Ihwi_w_xQ] Someone unearthed this old commercial starring Stephen Colbert before he became The Stephen Colbert. That the commercial is for a bank is mildy ironic considering his satirical posturing during these tough and turbulent economic times.
Semi-relatedly (so far as actors appearing in commercials before they were famous...), here's a young Jared Leto and Rebecca Gayheart in a Noxzema commercial.
[Via]