Matthew Miller over at nokiaexperts.com has made a summary comes from Nokia N900 User Guide. Although the original link of Nokia N900 User Guide has died, you still can see it here. These things look useful enough. You can see below.

Notification Light Color

N900 has the notification light color on its upper left corner. The user guide says that:
Orange – battery charging
Green – battery full
Blue – app triggered app (receiving IM, email or text)

Keyboard Shortcuts

Some examples of them:
·Ctrl + Backspace: If you are in an app this shortcut will bring you back to the visual task manager.
·Ctrl + C: Copy text
·Ctrl + V: Paste text
·Ctrl + X: Cut text
·Ctrl + A: Select all
·Ctrl + O: Open (if available)
·Ctrl + N: Create a new item (if available)
·Ctrl + S: Save (if available)
·Ctrl + Z: Undo (if available)
·Ctrl + Y: Redo (if available)
·Ctrl + F: Open search bar (if available)
·Ctrl + Right arrow: Move the insertion point to the end of the word
·Ctrl + Left arrow: Move the insertion point to the beginning of the word

Miscellaneous

Circle zooming: Double tap on a text will zoom it. You also can do it by spinning the finger clockwise. As for spinning it counterclockwise will zoom out.

Smart dialing using QWERTY: You have to type someone name and the contact will be filtered to that name.

The rest can be read here.