The clove hitch is a quick method of tying on to a pier or post. It is also possible to attach line to a carabiner (one-handed, which makes the knot more useful). It is not completely secure and should be finished with a couple half-hitches if you want this knot to hold reliablly.
Wrap the rope around the post (or carabiner clip) in an the alternating fashion shown.
Pull on both ends to tighten, add a half-hithc or two for ultimate security!