I know, I know - there's an actual site called Regretsy - which is actually quite entertaining in a sometimes amusing type of way, but which I personally feel I can't really fully endorse because of some of the subject matter that they choose to include.
When I think Regret-sy I'm thinking something AMAZING from Etsy that I procrastinated and second guessed myself about getting and subsequently.....missed out on *sulk*.
Like this shirt. Isn't it AWESOME?!