The Pokétch in Sinnoh had some really useful apps, some of which have never really been seen since, including:
Pedometer (useful for egg hatching, Vs. Seeker, and the Poké Radar)
Pokémon List (only games so far where you can see your current team and their HP in the overworld)
Friendship Checker (allows for a way to see friendship status of current team anywhere, albeit in an overly simplified way)
Day Care Checker (allows you to see what you've left in the Day Care and their current levels anywhere)
Marking Map (this is the best tool for chasing down wandering Legends EVER)
Move Tester (built in type chart)
Matchup Checker (the ability to see if two 'mons can breed is amazingly useful)
Pedometer (useful for egg hatching, Vs. Seeker, and the Poké Radar)
Pokémon List (only games so far where you can see your current team and their HP in the overworld)
Friendship Checker (allows for a way to see friendship status of current team anywhere, albeit in an overly simplified way)
Day Care Checker (allows you to see what you've left in the Day Care and their current levels anywhere)
Marking Map (this is the best tool for chasing down wandering Legends EVER)
Move Tester (built in type chart)
Matchup Checker (the ability to see if two 'mons can breed is amazingly useful)