Sibelius - I don't have to manually create individual parts from my orchestra scores or put a page into the trash bin just because I did one mistake.

REAPER, Pure Data and other - I don't have to slice and glue magnetic tape to create electronic music

Thunderbird - I don't have to handwrite letters and bring them to the post office

I think the more interesting question would be: which computer program hasn't saved me 100 hours...