[See update at bottom of article]
Currently my everyday light is a Magicshine MJ-818 which uses a 7.2 volt external LiIon pack, generally most people run it from the same pack they run their headlight from. If you already have a MagicShine headlight, the taillight with a Y power adaptor comes in at $30 and is absolutely unbeatable at that price.
For years, the standard self contained blinkie was the Planet Bike SuperFlash, and it's still respectable, but lately it has had competition and I believe at this point it's probably better for most people to look elsewhere. The Superflash costs $20 (there's a turbo version for $30, but is still only 1 watt and others are more powerful for right around the same price). Also the Superflash is known to not be completely waterproof and if run with alkaline AAA cells, not as environmentally friendly as it could be.
Especially good is that many lights now are USB rechargable with very long lasting LiIon batteries. I've seen many cyclists out at night with nearly dead batteries. With rechargable lights you never need be tempted to get that one last ride out of a set of batteries; just charge them up when they're getting low. USB recharging is good because almost everyone already has some way to charge via USB so you don't have to keep track of yet another charger, and not producing and shipping (and ultimately, throwing away) another charger is good for the environment.
Lately the Cygolite Hotshot has been touted as the best cheap light. At about $30 street it's pretty good. However, it's very "spotty" and even 10 degrees off center the beam drops off considerably. It is USB rechargable (via a mini-USB cable) and has an interesting ability to program the speed of its various flash modes to suit the rider.
The new contender that I'm playing with is the Knog Blinder 4V. I just bought a road bike and wanted something that would be lightweight yet powerful, and would look nice on my bike. The 4V fits the bill and surprisingly may be my favorite light right now, perhaps even beating out the Magicshine. It's just a bit more expensive than the other small blinkies at about $35 street price.
When viewed from directly behind:
Magicshine is by far the brightest. The Hotshot is next with quite a lot of light for such a little thing. The Knog is a little less bright, and the Superflash is a distant fourth.
When viewed at a slight angle (say 15 degrees, IMO the most important viewing angle since many cars will be to the side a bit, and lights are often not mounted exactly straight):
The Magicshine is still pretty bright. The Knog is just as bright as from directly behind and is close to as bright as the Magicshine. The Hotshot has dropped off considerably and is not as bright as the Knog. The Superflash has dropped off and is really not that bright at this point.
When viewed at a steep angle (45+ degrees to either side)
The Knog is the brightest, up to about 70 degrees to one side (140 degree spread) it's pretty much as bright as it is head on. The Magicshine is still OK, not quite as attention getting as the Knog. The Hotshot has dropped off badly at this point and the Superflash really isn't much worse here than at a lesser angle.
Here's a comparison with the lights bouncing off a white garage door.
Here's a comparison with direct viewing.
TL;DR - the Knog Blinder 4V is a winner. It is thin, it's USB rechargable, it looks great even on a road bike. It has a nearly totally flat field, showing just as much light out to the sides as directly behind it, and it's plenty bright in that range. It also has a range of blink modes that should satisfy most people.
The downside: It's seat post mount, and would be cumbersome to mount some other way. It needs to have some kind of seat post sized tube to wrap around, so if your seat post is hidden behind bags and you're not able to provide some other kind of thing for it to wrap around, you'll probably have to look elsewhere. It'd be nifty if Knog produced some alternate mounts such as a seatstay, reflector bracket or rack mount.
Another downside is the somewhat odd charging connector - it plugs in just like a thumb drive which is great but on some computers you may need to purchase a USB extension cable to plug it in properly. It just plugged into my PC just fine.
[UPDATE] - The Blinder 4V battery life is relatively short on all modes but eco-flash. This is not too surprising given how bright the light is, but for me, the light barely makes a week in the "organic flash" mode that I like. I'm used to charging my other taillights about once a month. This isn't a huge problem but keep it in mind - I was caught at the beginning of a ride with a dead light,which is better than having it just go out in mid-ride.