The Southernjets voluntary airport standby is a surprisingly good deal. It is up to a 6 hour window, and is used pretty low in the coverage process: after all regular pickups, reserves, and premium pickups. It is basically the last step before they reroute someone. If you get assigned a trip you have to be released as soon as you touch your base again or on the second calendar day, whichever occurs first. If you don't get used it pays a full day's guarantee (5h15') plus an additional 3h. Line holders will get the entire 8h15', reserves will have the 5h15' go towards their reserve guarantee for the month, but the 3h will be on top of guarantee. If you get a trip, it pays the value of the trip plus the additional 3h. Similarly the trip credit goes towards the monthly reserve guarantee for reserves but the 3h stays on top of guarantee. They can also be picked up as greenslips and all the usual greenslip additional pay applies.
They were offered a few times in my category so far this year, and basically went to area code seniority as a straight pickup.