This topic has probably come up here previously, so if there is an existing thread [MENTION=112383]MoD[/MENTION]s please merge.

Many countries require proof of onward (or return) travel as condition of entry. Often, a simple airline reservation printout is sufficient, but I prefer actually booking tickets that I can cancel with little or no penalty.

US-based airlines are pretty good about providing free cancellation within 24 hours, but this is often limited to tickets that originate in the US.

From what I can gather, United award tickets can be cancelled (and miles redeposited) up until departure, though I would never cut it that close. I've also heard that Priceline offers 24-hour risk-free cancellation on all airplane tickets. I'm interested on thoughts about these two, as well as others, including specific airlines or OTAs that offer timely refunds on revenue tickets.