Here are a couple that have served me well for the last five years.

Covering helmet vents
In the winter I cover my helmet with a layer of glad press & seal wrap. It prevents air from coming through the vents and chilling you. It's sticky and stays in place, but leaves no residue whatsoever. In the summer I cover the front vents so I don't look like my hair is doing the wave from spot blow drying.

Visibility & Illuminite
I can't stress how much I love this stuff. I'm on my second jacket, and also have a helmet cover, headband and light gloves covered in it. For those that are unaware, it's a super-reflective coating. It practically guarantees that you'll be seen at night. It only works when you are in the direct path of light back to the viewer's eyes, so it doesn't replace active lighting. www.illuminite.com It's not pretty, but I also cover a lot of the frame in 3m scotchlight tape in red and gray.

Wet
I keep everything in my commuter bag in ziplock bags, so they stay dry no matter what the weather. I also keep a full size garbage bag in case it's really pouring to line my bag with. It's also good to stash wet gear/stuff in, in case the ride home is dry.

First aid kit.
I keep 2 rolls of 3inch gauze and 6 4x4 pads and 2 quick clot sponges, in a sandwich size ziplock freezer bag. This material can stop an arterial bleed, and in fact, in their testing they had a 100% save rate on cows with severed femoral arteries. www.zmedica.com or www.quikclot.com It now comes in sponges rather than just granual form. If you get a really bad bleed, you just jam a couple of the sponges in the wound. They can literally save a life. They're not cheap (about $40 for a package of 6), but they last for years. You can find them at www.galls.com

Repairs
I ride a beater so I keep about two feet of duct tape wrapped around my seatpost for improvised repairs.