Â Couple things come to mind: if you may possibly need more gas in the near future (with summer just around the corner, I had to ask) you may be able to refill it instead of disposing. Local gas(oline) stations may be able to do it for you. Also, local gas delivery companies often have it all: refill, swap onto a full one and they'll take one off your hands for free.
Home Depot, Lowes, Walmart and just about every grocery store chain out there will do the swap onto a filled one.
But if you're determined to get rid of your cylinder, I would call a local gas company (feed "propane delivery + <i>your ZIP code</i>" into your favorite search engine) - that's a sure bet.
Before you make a decision to dump your cylinder though, check out if it may be a part of this recall:
The recall specifically calls for propylene, MAP/Pro and MAPP 14.1 cylinders but some people use propylene and propane interchangeably (and I ask your forgiveness if you are not <strong>that</strong> kind of a person :) ) so I thought it's worth mentioning. In fact, some 29 million cylinders are affected, so if you ever (Oct 2004-Jan 2012)Â bought one, you can seek a refund. Could be a better alternative to disposing...