In the original Half-Life, you had all kinds of weird guns with all kinds of awesome abilities. One of them is the Tau Cannon, or the Gauss Gun.

Now, your normal jumps are... well... normal for a wiry scientist in average physical condition.

But, hold down the alternate fire button (usually the right-mouse button) for a few seconds to build up a charge, then aim it straight down and let go. The kickback will launch you into the air. How high? About this high:

High enough that you'll injure yourself when you land.

Awesome.