I was turned on to xcape by evaryont in a comment on my entry on my Backspace/Delete X key mapping shift. What xcape does is, well, let's quote from its own readme:

xcape allows you to use a modifier key as another key when pressed and released on its own.

A modifier key here is things like Shift, Control, and Alt (and CapsLock if you turn it into a modifier, such as making it another Control). The common use of xcape is by vi people to make one of those keys act as Escape when it's tapped, so they don't have to make the long stretch off to the top left of the keyboard for a key that they use all the time; instead they can tap something much closer.

(At the same time they don't lose the normal use of a valuable modifier key the way they would if they completely turned one of the modifier keys into Escape with, say, xmodmap.)

It's not clear how xcape works from the manpage or the readme. Before I started reading the code (it's short), I had concerns that it actually intercepted the modifier key and did weird things with it, which might interfere with other programs. This is not how it works. Instead, xcape passively listens to all keyboard events; when it sees a press and a release of the modifier key alone fly by within its time window, it injects a synthetic key-down and key-up event for your chosen additional key. No existing events are touched, only new ones added.

(Xcape is listed as Linux specific, although it might not be; it only seems to use the X 'record' and 'XTest' extensions, and I think they're generic. The record extension is used to monitor key events, the XTest extension to inject the new ones.)

What I'm using xcape for is a bit different from usual. Dmenu is a core part of my environment, and I have my window manager set to bring it up when I hit F5. F5 was in an easily reached location on my old keyboard, but on my new keyboard it's moved just enough so it's no longer a casual, rapid tap. So I'm using xcape to make tapping the CapsLock key (which I normally use as a Control key) also generate F5 and thereby bring up dmenu. The CapsLock key is of course in an extremely convenient and easily reached spot, which is great for this.

In general this works and achieves the goal of making bringing up dmenu be a fast, easy thing. The one drawback to reusing CapsLock is that I sometimes activate dmenu accidentally during normal typing; evidently I can plan to type a control character but then rapidly change my mind without thinking about it, which creates a CapsLock press and release close enough together to trigger xcape. If this turns out to be a long-term annoyance, I'll probably shift dmenu to being triggered off the much less used actual right Control key.

(This keyboard also has Windows keys, so I could go all the way to making the otherwise unused left Window key trigger dmenu, which wouldn't need xcape at all. But on the whole I like being able to call up dmenu so easily and casually, so I'm inclined to keep things the way I have them now.)

It's possible that someday I'll add an xcape mapping for Escape, but I'm extremely used to hitting Escape in its current location now (it's basically a reflex action at this point) and I don't really find it a problem. Still, I acknowledge that I may be missing out by not doing so and devoting the time to acclimatize to a new Escape location.

(I'd probably put Escape on the left Shift.)