I think my favorite "buy for life" purchase so far has been Backpacking Light. I picked up the unlimited lifetime membership over a decade ago and while I don't use it often, when the need comes for detailed knowledge on hiking and camping that's the go-to source. It's so handy!
@tomasino agree 100%. My $5 REI membership, bought back in 1995 has been good. And, even though I'm only about a week into it, the $36 lifetime membership over at sdf.org is pretty cool!
masto instance for the tildeverse