If you downloaded an Xbox Original title from Xbox LIVE be sure to check out the pages for each game on Xbox.com. Each games page has a link to a PDF of the games manual, as well as a controller map to tell you what is where:

Crash Bandicoot: The Wrath of Cortex (Manual PDF) (Controller Map)
Crimson Skies: High Road to Revenge (Manual PDF) (Controller Map)

Fable (Manual PDF) (Controller Map)

Fuzion Frenzy (Manual PDF) (Controller Map)

Halo  (Manual PDF) (Controller Map)

Indigo Prophecy (Manual PDF) (Controller Map)

Psychonauts (Manual PDF) (Controller Map)

Also, (and I have not seen this mentioned anywhere yet) grab the free theme and picture pack that each new Original has available.
The quickest way to find them is by going to the Marketplace blade>Game Store>Xbox Originals then choose a title. From there you can download the free theme or picture pack, as well as purchase the game.