One life-saver that I have is my Rechargeable Air Horn. It's great. The week after I got it mounted, I was able to scare-to-a-stop an old man trying turn and drive through me to a parking lot! I also use it when people drive in the bike lane. The front view you can see that I ran out of space to put the refillable air resevoir (conveniently filled by a bike pump). So it is mounted on my front fender.
The 1200 Lumen light was a great Christmas present from my buddy (an inexpensive $40-50). They're WAY cheap too! These lights are 1200 Lumens, come with a lithium battery, bike mount, and helmet mount. The light that I got has 3 light settings (High, Low, Seizure Mode). It works great, except the seizure (flashing) mode will probably not be something you want to use. I can honestly imagine car accidents because of it. Another similar-priced 1200 Lumen light model has 5 modes. Its flashing might be more reasonable. Regardless, my 3-mode light works great with High and Low settings.
I like having this light mounted to my bike. My friend has 2 and has one of them on his helmet. Whichever one is mounted to the bike, you should get the horizontal light diffuser. It's ~$5, and turns a spot light into a horizontally diffused light. Then you can use your helmet light for a long-distance spot light.
I have another light that I like to use, a Cygolite Expilion 250. It is also a Lithium Ion light. It is rated for 250 Lumens and works great for a point-light. Unfortunately I think it's gotten expensive since I bought it. It is very bright though and conveniently charges by USB. I usually bring my helmet into work with me, so if my light is low, I can charge it.
On my road helmet my other great safety device is my rear-view mirror. It comes in 2 versions, to either attach to your helmet or your eye glasses. I have bought both versions (compact version is for attaching to your glasses). I don't wear glasses enough to use the compact, so now I just use the "original" version. Both work great. They are just out of regular view so that they don't obscure your vision. But they keep your situational awareness up much higher. Even though I have the mirror, I still recommend double-checking over your shoulder before turning into the traffic lane next to you.
Another essential that I think is needed is a Shoe and gloves dryer. I spent over $50 buying shoes for my bike, and I don't want to buy them every year due to molding and smelling and such. You have to be careful when you buy these. You want the ones that have BOTH Heat AND Air. My buddy bought two. The first one just blew regular air around in your shoes. The second was a "silent" dryer, and did not blow any air, it just heated up the air in the shoes. Mine does both, and is much quicker at drying because of this. I don't care about it being silent...it's in the garage.
I have some cheap ear behind-the-ear muffs that I like to wear, even when the weather is up in the 90s (I'm only going a few miles). When I ride my bike to work with my buddy, the wind across my ears is pretty noisy and so I can't ear my friend's conversation or the car behind me. When I put on these muffs, though, the wind noise is almost completely gone. They're great and were a few dollars in Korea.
Finally, I have a toilet-bowl cleaner and tooth-brush for cleaning my bike off of mud. I also have some reflective pant leg bands. The leg bands are to keep my pants from getting caught up in the chain...and being reflective is a plus. They were ~$5 at a local bike shop.
