Double Fisherman's Knot
This can be used for joining ends of rope together. Done correctly, it is very strong and relatively low profile. We most commonly use it to make loops for prussiks, which in turn are useful for all kinds things: tying in to fixed lines, escaping belays, ascending a rope, crevasse rescue, and so on.
How To Tie a Double Fisherman's Knot
This is a nice illustration.
Here are some images from wikimedia commons: