I went to the page and my 21st century brain ignored everything apart from immediately looking for swag photos and price tag...
Of course, I saw it and was blown away (I only spend £15/$25 on a t-shirt max) and in a split second ruled it out...
But a second went by and I actually realised saw donate button and then the big header about contributions to letsencrypt etc....
If I look back at the (I guess) 6 years of using them.. across maybe 60-80 personal domains across 8-10ish TLDs worth of certs... If letsencrypt hadn't been a thing, I'd have still been paying £50/domain or £150/wildcard root domain cert per year. Realistically, for my own personal projects, they've saved me close to $10K. Ignoring the fact that I don't need to renew tens/multiple-tens of certs a year and the hassle involved with that.
I definitely think I owe this to them and will definitely be buying something and hope others in the same boat do too :)
It's sort of wild to thing of a time now without being able to get a new project up-and-running by just sticking up a DNS entry and 3 seconds later having a valid SSL cert attached :)
Edit: Bought something :D