My favorite girl detective is turning 80 this year: Nancy Drew.
I loved reading all the Nancy Drew mysteries as a kid and like Sonia Sotomayor, I went through a "I want to be a sleuth when I grow up" phase. Obviously, that didn't happen (yet--there's still time for a mystery to drop in my lap, I suppose).
If rereading The Secret of the Old Clock or The Clue of the Dancing Puppet isn't enough celebrating for you, Fred Flare has tons of Nancy Drew merch, from skeleton key rings to ND stationery to clothes for all your dress-up needs. Personally, I think I need to get my hands on a copy of Nancy Drew's Guide to Life.
(Thanks to Galley Cat for the heads up)