Option + Left Shift = Find
Option + Right Shift = Toggle bright/dim
Option + P = Adjust brightness 

(although I typically use
Option + Right Shift, mentioned above,
which is a much easier way to dim the screen quickly)

Shift + Space = Tab
Option + Up/Down = Scroll (in some apps, notably the Launcher)
S + Alt + Alt + Alt + Alt + Space = Graffiti shortcut symbol

As far as shortcuts, you can use the built-in shortcuts like dt, dts, lu, etc…, but you cannot add your own. For that, you’ll want the freeware program Shortcut5. Be warned though that this app when installed appears as a Preferences panel; not in the Launcher, and that it defines it’s own style of shortcuts that start with a period rather than the traditional shortcut symbol.

For easier scrolling, it’s nice to have Butler ($12), which among other things, allows you to scroll (and switch applications) with the volume rocker. Butler does a ton of other useful things too – alarms, alternative KeyGuard, etc…

For easier typing of capital letters and punctuation, it’s nice to have KeyCaps650 ($10 suggested donation). This app makes it so that holding down a letter key capitalizes it and a quick double press of a key causes it to display the character associated with the Option key. Only problem is you have to learn to type double letters a tad slower when you really want them so that they don’t get changed into punctuation.

Holding down the Center button defaults to showing some alarm dialog that I don’t find useful. Instead, I installed
KeyguardTime+ and configured it to map the Center button to showing time, date, and battery level (sort of like ClockPop5 on my old Tungsten T2).