You will be much happier on your bike if you keep the tires correctly inflated. It is amazing how much of a drag a low tire can be, in addition to putting you at a greater risk of punctures. I keep a pump at my office, because no one sees it under my desk and I have more room here than at home.
A Bike Lock
All locks are vulnerable, consider them only to be a deterrent. There are two general types:
- Cable lock - intrinsically more flexible, faster to cut through
- U lock - generally stronger, harder to find suitable locking locations
A Tool Kit and Spare Tube
I carry a spare tube, and usually a small pump. On occasions when I don't have the pump I'm relying on a kind cyclist with a pump to stop if I'm distressed, but I'd never expect even the kindest of comrades to give me a tube.
For my minimalist tool set I carry plastic tire irons, regular and Phillips screwdrivers, a small adjustable wrench, and a small Allen set. I don't try to patch a tube while dealing with a commute.
Headlights and Taillights
Lights are legally required after dark in Massachusetts, and a good idea wherever you are, laws or not.
The old-fashioned romantic in me would like to be using dynamo hubs to power my lights, but that hasn't happened for me yet ($$). In the meantime I've been very happy with a rear red blinkie and a handlebar mounted headlight. These run on batteries and are very safe. The handlebar light lives in my bike bag when not on the bike. The rear blinkie is mounted on my helmet, so it is theft-protected as well.
My philosophy on helmets is that they don't hurt and they might help. I don't want to live with the regret of not having done all that I could to stay safe in the unfortunate event of an accident. So I wear one. The only difference between an expensive helmet and a cheap one is how it fits, so buy the cheapest helmet that's comfortable on your head.
I love my fenders. I do ride in the rain but I'm more likely to ride right after the rain. My fenders keep the road grime and wet from splashing back up onto me.
I have a rear rack on which I clip my bike bag. It also works well for carrying parcels to and from the post office and even bulkier options, like my new living room carpet.
Almost a requirement, bags which attach to your bike allow you to carry things without having them weigh down your back (and arms, and wrists). You can leave them partially packed with tools and a spare tube. They come in many styles depending on your bike and your needs:
- Rack bags and panniers
- Handlebar bags
- Underseat wedges (usually quite small)
Whatever you happen to be wearing today! (with possible accommodations if you live in a hot climate and your clothes are delicate) Bicycles work just fine with no special clothing.