Oh man...
The standby ones! They often seem to come with so many clauses that I've never even really gotten a chance to THINK of when I'd use one. I did a SDC once, and they only did it because I took a VDB because of it.
The other ones that get me are the "we'll let you buy a seat even if there are none" type deals. I'm sure there are some business pax or whatever that need a certain flight at a certain time and don't really care the cost, or maybe are on a flight pass, but I'll take the connection at YYZ and a couple extra hours to spend $800 instead of $2200 on the direct sold out/oversold flight.