When it comes to getting various notifications while your device's display is off, a built-in LED is useful. However, it's far from perfect. Sure, you can customize each notification with something like Light Flow, but that doesn't let you know the message contents at a glance. And, of course, there are also devices that don't have an LED at all, like the Nexus 7 for example. If you're looking for more out of your notification experience, Knock²+ may be the answer.
Update: Apparently the app isn't compatible with the Nexus 7 right now, but the dev is working out the kinks and should update the app "soon."
Like others before it, Knock²+ essentially provides a lockscreen popup when a notification is received, so you need only glance at your device to see what the ping was about. Where it differs from many other solutions, however, is in aesthetics – this thing is beautiful. It's so clean and minimal.
It's also quite customizable, and the dev appears to be adding more features and supported apps all the time. Even a quick look through the reviews up to this point reveals excellent developer support, as many users have reported nearly-instant bug fixes and additional support for requested apps. That type of support is always sought after, especially for paid apps – after all, when users shell out a few bucks for an app, timely support is expected.
Speaking of, Knock²+ will set you back a two dollar bill. If a more in-depth notification experience is something that could make your life easier, hit the widget to check it out.