http://www.trustedreviews.com has a very comprehensive list of what is available. I have copied them here for convenience.
Add full stop and space by tapping space-bar twice
When you are typing any text, just try to tap the space-bar twice and see what happens. WP7 automatically changes the first space into a dot, so you end up with dot and space. This is much quicker then aiming and tapping to the small dot-button
Tap and hold different buttons on the keyboard
You can get lot of nice gem jus by tapping and holding specific buttons. Here are some of the most useful for me:
Write punctuation without switching to "punctuation-layout"
If you want to write one of these chars
?!:- you usually tap the
&123 button in lower left corner of the keyboard. But you can also stay on the standard keyboard layout and tap and hold the dot button and pick on of the chars directly from there.
Tap and hold the backspace to quicker deletion
If you want to delete more text and selecting is hard, you can tap and hold the backspace. It will delete by chars from start, but after a little while it will delete by words.
Tap and hold a language button to see all languages in use
If you have more then one keyboard language chosen, you don't have to iterate through all of them to choose the right one, just tap and hold the language button and all languages will popup so you can select the one you want
Use capslock for long text in caps
If you want to write something in caps (part of password for example), you can switch CAPSLOCK functionality on by tapping and holding the shift button or by double tapping it. Tapping it again will turn capslock off.
Tap and hold other buttons on your keyboard to see some hidden gems
Some of the buttons that are hiding something:
^<>=-|. Just try it and find the ones you needed. When in url or email textbox, tap and hold the
.com button to see other useful domains.
Don't touch the buttons in popup
When you tap and hold any button on keyboard for extended choice of chars, you can move your finger anywhere on the screen as long as you stay "connected" to the screen with your finger. This way it is very free to choose button from popup as it matches the right button just by the vertical intersection.