What's the best thing you've ever bought... That's completely useless in all situations?
For me, it's this: http://gadget.brando.com/wildlife-photography-with-tactical-four-reticle-sight_p01341c073d003.html
You can't calibrate it's position, it doesn't actually give you a good idea of what you're shooting, and it makes you look like a weird combination of geek, tool, and worrying gun nut.
But by god, best 50 dollars I ever spent.
For me, it's this: http://gadget.brando.com/wildlife-photography-with-tactical-four-reticle-sight_p01341c073d003.html
You can't calibrate it's position, it doesn't actually give you a good idea of what you're shooting, and it makes you look like a weird combination of geek, tool, and worrying gun nut.
But by god, best 50 dollars I ever spent.