Full disclosure: I can, technically, live without this stuff. But I wouldn't want to. Don't deprive yourself. I ride every day. I know what I'm talking about.
My favorite bike accessory, and one that I have recommended to all of my friends and family is the Planet Bike Blinky Super Flash 1/2-Watt Blaze LED Plus 2 eXtreme LED Rear Bicycle Light. It is, without a doubt, the brightest blinkie out there. I have two. Blind people squint at these things. If a car hits you from behind while you're using one of these, it was aiming for you. It's also available in a Planet Bike 3040 Superflash Tail Light and Blaze Headlight Light Set with a damn decent headlight. You could spend less and get a dim, crappy blinkie but that would make you an idiot.
My next favorite bike item is the Eco Friendly Polar Insulated BLUE 24 oz Water Bottle-Made in the USA-PBA Free. It keeps your drinks cool WAY better than regular water bottles. If I fill it with ice water, before I leave the house for work, it's still icy cold at lunch time. Ever had a blended margarita after a long ride? You're missing out. Get one.
Next on my list of faves is the Delta Airzound Bike Horn. It's loud. Like, keep away from children loud. Drivers will pull over for you if you blow this behind them. Dogs will whimper three blocks away. You need it.
This one is an oldie but goodie. Carmex Original Lip Balm Click Stick 0.15 oz / 2 count. This stuff is saved my life. Twice.
Watches aren't technically bike gear, but they do let you know if you're late for work (and thus, need to ride faster). I love, love, love my Casio Men's Classic Black Digital Watch #F91W-1. My grandpa had one. I've rocked this watch since I was 9. I bought one for my 4 year old nephew and he likes it too. It's small enough that you barely notice it, but it's easy to read and has retro charm. And it's eleven bucks. Duh. Buy two. Or ten.
The last thing that jumps to mind at the moment is the AeroPress Coffee Maker with Bonus 350 Micro Filters. Starbucks sucks. Seriously. Honestly, it's aweful. This bad boy makes a damned respectable pseudo espresso and the whole thing only costs as much as two vente quad soy pumpkin spice lattes with no foam. Buy one for yourself and one for your riding buddy, that way you're both good to go.