The digital life suite of iTools / .Mac.
iSync was a really great tool. It’s silly to think about now, but back then, syncing data across devices was a painfully manual process. iSync just let you plug a phone or iPod in and sync your contacts, calendars, and notes.
iPhoto was also my most used app even though it got insanely slow as you added more photos. And iTunes was critical for my senior year of high school. I kind of miss Ripping and burning CDs.