Giving music-related gifts to guitarists is not easy - often the gift giver doesn't know exactly what a guitarist already owns. The following items have been chosen not only because they are light on the pocket book, but because many of them are things many guitarists don't own - or if they do, could always use more of.
Here is a dozen, good quality, middle-of-the-road gauge (thickness) guitar picks from Dunlop. These picks can be used on an acoustic or electric guitar.
This one tips in at a little bit over the $10 mark, but a tuner in this price range is still a bargain. The Snark tuner clips on to the headstock of a guitar, and uses vibration rather than pitch to tune the instrument. This makes tuning in a noisy room much easier. This can be used on an acoustic or electric guitar.
All guitarists should own a slide. Although absolute beginners will frankly sound horrible using a slide at first, having one around to experiment with makes sense. This Dunlop steel slide is a standard issue model that is exactly what a guitarist needs to get started. This can be used on an acoustic or electric guitar.
This is frankly not my first choice for capos (for that I'd look to the Shubb), but if you're looking for a super cheap little capo, the Kyser performs perfectly well. This can be used on an acoustic or electric guitar.
Here's a neat little two-in-one device that guitarists will love - a combination string winder/string cutter. This allows you to change strings on your guitar using one tool (traditionally, many guitarists will carry both a pair of pliers and a string winder for string changing purposes). This can be used on an acoustic or electric guitar.