Samsung Galaxy S II - Tips & Tricks

Pinch at the Calendar's month view to go into year view.

Pinch twice in the stock browser to see your other windows.

With the TouchWiz launcher, pinch at the homescreen or app drawer to go into thumbnail view.

With the TouchWiz launcher, navigate the homescreen or app drawer by tapping or swiping those little dots on the bottom.

In the call history (log) or contacts, swipe left to send SMS or swipe right to call the person.

Tap and hold the notification bar until the bottom shade disappears. This should take about 1 second. Then  swipe laterally to adjust screen brightness.

Press Home + Power button to take a  screenshot. Root not required!

Go to about:useragent in the stock browser to amend the user agent string. Note that the change only applies to the current window/session.

Go to [Settings > Accessibility] or [Settings > Call > Call answering/ending] to toggle on "The power key ends calls". You may also toggle on "Answering key" here to use the Home button for answering calls but that's a personal preference.

Go to [Settings > About phone] and spamming the "Android version" with taps will give you an Easter Egg.

In the Calculator app, slide down the number pad to reveal previous calculations.

Hold the Menu button to bring up contextual search (equivalent of pressing the Search button on other devices).

Press the Home button twice to bring up Voice Command.

Hold the Home button to bring up the 6 most recent apps as well as shortcut to the Task Manager.

In the Camera app, press the Menu button and tap Edit shortcuts to have quick access to up to 4 settings.

In the Camera app, use the volume rocker for digital zoom.

While watching a video with the default video player, press the Power button to lock the screen while keeping it on.

In the Camera app, you can tap the screen to focus on an area other than the center. Alternatively, you may frame the object you want to focus on in the center, tap and hold the on-screen shutter button and then move to reframe the shot to your liking. The focus will lock. Release the shutter button to take the shot. I find this to be faster than using tap-to-focus.

Pressing *#0*# in the phone dialer will bring up LCD test.

In the Email app, pinch to collapse all emails by date.

With the Samsung Keyboard, if you enable "Keypad sweeping", you can access punctuations by swiping laterally across the keyboard.

Personal app recommendations to improve usability.
Dolphin Mini. The stock browser supports hardware acceleration which helps with rendering speeds. But Dolphin Mini is no slouch in this area. I think the ability to permanently set the user agent string and to go full-screen greatly improves the browsing experience.
LauncherPro Plus. Fast and smooth animations. Useful and elegant looking widgets. Highly customize-able. I just find it better in every way than the Touchwiz launcher.
Widgetlocker Lockscreen. Allows you to place homescreen widgets on your lockscreen. Just like in HTC Sense 3.0, allows you to launch applications from the lockscreen.
SugarSync. Auto backup all your phone's photos taken with the cam to the cloud and have that auto sync with your PC.