#1
So I was lurking on craigslist earlier and found somebody selling a pedal called a Gig Buddy. I'm guessing it's just a one-off pedal since I couldn't find anything else about it, but from what I read it checks your cables to ensure you're not running any faulty ones before your rig.

In other words, you plug both ends of the cable it, and 2 lights light up if the cable is okay to use.

Would this be difficult to build/be worth it?

I have no prior pedal-building experience and have some electronic/wiring knowledge.
#2
It's probably just a continuity checker. You'd want to measure the resistance of the cable, plus the capacitance of the hot to ground as well to really be able to use it. You'd be better off with just hooking up a couple jacks, and zero-ing out their resistance before hand. The only issue is the jacks would lose their conductivity slowly over time. So you have to zero them out/ clean them before testing a series of cables.

Get a plastic radioshack box, and two 1/4" jacks. And measure them yourself, just my .02 though.
#3
Hmm... you can always buy a cheap Digital Multi-Meter to do that, but having a dedicated pedal would be pretty cool. I guess there's a couple of different ways to go about it.

One would be simply having the cable complete a circuit with an LED and resistor.

Of course, in a cable tester you'd want it to be a bit more advanced...