There aren't many things that can give you a fundamentally new way of playing the guitar. There's a slide, for the fretting hand, but what of the picking hand? Well, there's the Ebow.
The Ebow is a plastic device looking something like a bent capital L, or perhaps a tiny wood-plane. You switch it on (a 9v battery is required) and hold it over a guitar string to make the string vibrate constantly, to provide an even tone. There are in fact two switch settings, one of which gives more "harmonic richness" than the other. (To my ears it basically adds an octave above the note you're playing, something like the sound you get holding a note and letting it feedback.) The Ebow's aim was to produce something a bit like a violin bow's smoothness on a guitar string, and it seems to work. (Go to Ebow's website for some videos of people using it, and you'll get an idea. Or just listen to Bowie's "Heroes".)
I had a basic idea of what the Ebow was before I got it, but on removing it from the box and trying it out, I learned a few things. One is that the Ebow sits above one string at a time (I thought it might do multiple strings at once). It in fact has two plastic guides (grooves) that sit on the strings either side of the string you're playing, thus positioning the Ebow in the right place, but also muting the strings either side. (This also means it takes a little bit more practice to sit the Ebow over the top or bottom E strings, as there, they'll only be sitting on one support string.) These guides also make it more difficult to move from string to the other. In fact, at first I thought it would be impossible, till I saw some of the videos on Ebow's website, and realised you can make some quite wonderful arpeggio effects by moving the Ebow quickly over all the strings. It just takes a bit of practice.
So, the Ebow can add a whole new string to your bow (pardon the pun) as a guitarist. Like all things guitar, it benefits from a little bit of practice, but you'll still be able to do some lovely long-sustain single-string playing with the Ebow straight out of the box.