Can I make hot corners require a mouse click to be triggered? I love the "Hot corners" concept. However, it happens very often that I trigger them accidentally.

Is there a built-in way to add a "click" requirement to a hot corner? ie. I have to move the mouse there and then to left-click.

(This would also help recreate Windows 7's "show desktop" behaviour, which is cool for people who recently switched from there to OS X.)
Modifier Keys
or keyboard click ;)
You can hold ⌘, ⇧ or ⌥ when selecting a hot corner.
You will need then to use the modifier key and the hot corner to trigger the action.

BetterTouchTool
While you cannot use hot corners, you can use a single finger gesture like this: