I picked this up earlier this week with a few other things, but not sure what exactly it does (or how to use it).  Is it primarily for tracing phone or computer network cables, or can it be used for most any wiring?

Signal injector and tester for tracing lines

Yes.  It's commonly called a "toner".

Technically, it can be used for any copper cable / wire.  I wouldn't necessarily recommend it for trying to trace electrical cabling back to a load center.

Fluke Pro3000 Analog Tone & Probe.
http://www.flukenetworks.com/datacom-cabling/installation-tools/Pro3000-Analog-Tone-and-Probe

Manual:
http://www.flukenetworks.com/findit/9820562

Can be used on any wiring.

Attach tone generator to wire, use wand to locate wire.

Bonus: If you briefly short the wire together that you think is ringing, it will change the sound being made. Handy for verifying a wire when you have a whole bunch of them together and want to be 100% sure.

I have the same exact toner.

Also handy for testing continuity. I use mine to verify audio cables that I make.

ETA: Make sure to put the caps back on the clips when you are done using it. Otherwise the alligator clips can short together when it is stored and drain the battery in the tone generator.

Good old fox and hound.  Much nicer than my 20 year old set. I bet you have multi tones and cool shit on that one.

Pretty handy wire tracer.  I use mine often.

I use mine to trace romex in the walls in my house.  Works pretty good for that once you learn how to use it.

Hey, check out this thing I bought.

Neat.  What is it?

I was hoping you knew.

As others have stated, it is a toner and probe. That particular model is designed for Class 2 wiring (low voltage) which is usually a DC circuit. It is not designed for household AC circuits, but could be used for them if they are not energized.

The toner is also capable of indicating shorts and if the circuit has voltage on it by using the indicator lights on the toner. It can also supply talk voltage (talk battery).

You can also use the sounds the toner makes to tell you if the wire pair it is on is good or bad by using a test set. Listen to the sound through the test set on monitor. When you go off hook, if the sound gets louder, the wire should be good. If the sound gets softer off-hook, then the wire pair has an issue -- usually an open in the circuit.

Did this come along for the ride with some other items or did you get it intentionally?

I'm trying to locate my toner/tracer to trace/check phone wires at a likely new office location next week. Could I talk you into a loan/sale?

If you need to make the signal stronger, say follow a wire through a wall or find which large bundle of wires your particular pair is in, hook one side to ground. This will make it bleed everywhere so it's not useful for isolating, just getting you in the neighborhood. You can also hold the probe with your finger touching the tip and your other pointer finger swiping large numbers of 66 blocks for example. The tip is plastic but it is conductive, you can still short contacts and affect circuits. The probe may have contacts that you can connect your butt set to. Put it on speaker and go off hook. This will allow you to amplify very faint tone that you might otherwise miss. Newer butt sets lock out/won't go off hook if they don't detect voltage or if they think they sense data (such as a ground hum) so never get rid your older ones.

Just sent you an email...

Handy to have and last forever. I've got different ones, but my toners and wand are 20+ years old and still work fine. I've got a 1960s or earlier rotary test set. I used all of them to set up a an old school phone recently. I'll probably use the toner and wand tomorrow to identify some cat6 cables at work.

Reply Sent.

Now to look for one of those 'older' buttsets that were referred to above. 

Works really well for that.  I have found wires within 1/2" of where the toner said, under drywall.