One of the most frustrating things about a lot of classes is that they might have a really cool ability, but it's only useable 1/day. Enter the Squire of Legend (Reikhardt), an early entry PrC from Complete Champion who can grant an ally one extra use of an Ex or Su ability. Take leadership (who doesn't?) and by level 8 your cohort can let you use that ability 2/day. Now admittedly, 2/day doesn't strike a lot of people as the bee's knees, but for me it certainly opens up a lot more potential. When I only have 1/day, I suffer from the "maybe next time" mentality and never use it, thus a lot of potential PrCs get looked over. But twice a day? Now I'm not afraid of burning the first use, and once I've done it once, I'm more inclined to use the second one (it's a psychological thing really).
So preamble aside, I'm asking you guys to help me out and list that classes/PrCs have really great Ex or Su abilities that are only useable 1/day.
I'll start with a nice one:
Fatespinner: target takes a -10 or +10 to it's next saving throw