1,117 colleges in 50 states and D.C, about 11.2 million students. Free seat alerts by email at every one of them, no app needed: search a class, pick the full section, get told the moment it opens.
The iPhone app pushes the alert the second a seat opens, builds four ready schedules from your degree audit, and ranks sections by real grades. Free; a $4.99 Fast pass per semester makes alerts instant.