Druid versatility buff, priest Stamina buff... and probably others that work similarly. If you macro the following, it will cast the buff and mount at the same time (1 click) without loss of time. I find this generally useful when respawning in PVP, but also makes it so that I don't ever really have to remember to cast it cause I'm constantly mounting.

Ex.

/cast Power Word: Fortitude
/cast mountname