I have two.
1. Baskets, or in my case a waterproof box called the Donkey Boxx. Having the Donkey Boxx has completely changed bicycling for me. It's large enough for me to hold my bulky waterproof backpack (for years I wore my backpack on my back). Additionally, the Donkey Boxx is always there for me for impromptu shopping at grocery stores, etc. Finally, it's waterproof. If it's raining outside I put my helmet in the box and it stays completely dry. Sadly, it appears that the Donkey Boxx is no longer made. If anyone knows where I can get another one, please let me know.
2. Full chain cases. This only applies to internal gear hub (IGH) bikes, but full chain cases are nothing short of a miracle accessory. My bike spends most of its life outdoors, exposed to the rain, and I do a lot of biking in the rain. Having the full chain case, no gunk gets on my chain. After 8,000+ miles of riding I'm still using the original chain (still hasn't stretched hardly at all!). The sprockets remain in great condition, and get this ... I've only lubed the chain three or four times in two years. And I haven't made a single adjustment to the gears or anything. I do plan to have the hubs overhauled soon though. Riding an IGH bike doesn't compare to derailleur bikes when it comes to speed or climbing up hills, but they require even less maintenance than single speed bikes. If someone can figure out a way to fully encase a derailleur drive train, bike maintenance will be drastically reduced for the masses.
