Well for now this is still a rumor but it seems in the next Xbox Live update you will be able to purchase original Xbox games published by Microsoft on Xbox Live Marketplace.  The games will cost 800 points and so far the titles include Halo: Combat Evolved, Fable, Crimson Skies, Burnout 3 and more.  I think this is a great idea and has me wondering if these games have been modified to have achievement points.  Now I own Fable and Halo but for some they probably never played these games.  I would assume there would be the typical demo model so you can download the trial for free and then have the option to buy if you like the game.  Nintendo started this pattern on the Virtual Console and now Microsoft and Sony(why else is the 40GB PS3 not backwards compatible) are hot on their heals to improve on the idea.

Update:  This is no longer a rumor.  Microsoft has a press release confirming the fact you can download Xbox Originals.