Ask HN: Bicycle hacks?

I'm a software engineer, but also a committed recreational cyclist. I'm sure there are others out there like me, and I'm wondering what kind of cycling related hacks members of the community have performed. Whether it's to increase performance, augment carrying capacity, find better (ie, safer, less-trafficked) routes to your destination, a total redesign of the bicycle as we know it, or just a cool modification, what have you seen or done?