Rhythm Sight Reading Trainer is a great way to practice sight reading rhythms on the go. There are more than 200 fixed exercises; the more difficult ones include rests, ties, syncopations, duplets, triplets, quadruplets and swing eighths (on iPad 100 longer exercises, because two iPhone-exercises are combined into one due to the larger screen).
Extra exercises for 4/4 (incl swing eights), 6/8, 2/2 and 4/2 time via a random generator (thousands of exercises). 22 difficulty levels, including a level for absolute beginners and a level to get experience with written music that violates the imaginary bar line rule.
Rhythm Sight Reading Trainer is universal and currently $2.99.