The harry one has already been posted, but not completely.

In Harry the handsome executive, press x. A box will come up with seeminglyu random poetry or something. This is really a quote from the They Might Be Giants song Mr. Horrible on their album Flood (Elektra, 1990)

Also in Harry press b (I think) for a scrolling credits box.

In Deimos Rising, press f for a two player demo (clicking 'demos' only shows one player.

In barrack, press x. A little poem will appear.

In pillars of garendall, press c for credits or l (not i but l) to open a file.

In Slithereens, press x. A box with a very cool poem/song with the authors picture next to it. All right!

In Mars Rising, press x for a John Donne quote. Press i to take a screen shot. (This only works on the title screen, so its rather pointless.)

In Bubble trouble, press x for a hilarious poem thing, and z for an emerson poem. Press l for a level select (it must be registered for this.) THe first time it will say 2-23, but if you beat 23, it will say 2-24, and so on.

I'm sure there are more such cheats in other games that i don't have. Try them all!