Brake light flashers (along with headlight modulators) are popular on motorcycles, they have been for a long time. I'm hesitant to recommend them for cars, partially because the only way us bikers stay safe is by keeping one step ahead of cars in terms of lighting, if you get brake light flashers, we'll need to make up something new!
They are easy to make with a PIC microcontroller, if you are handy with them, or you can buy them for not much money.
Here is a good source, they sell a lot to the motorcycle world, and are about 30$. I'd recommend the one that flashes 4 times in 3 seconds, then stays fully on.
http://www.comagination.com/bflasher.htm
Legality? It differs by location, but here is a general brakedown (pun intended).
Having all your brake lights flash constantly is probably NOT legal in your area.
Having just your third brake light flash constantly is probably NOT legal, but is probably subtle enough to get away with.
Having all your brake lights flash a few times, then stay on solid is probably legal, but might attract some attention.
Having your third brake light flash a few times, then stay on solid is probably the most legal.
Its important to note that these aren't 'strobing' your rear lights. Strobe has a very specific definition when it comes to automobile lights, and is generally illegal, because it emulates emergency vehicles, and can cause target fixation. Flashing, or modulating, is more legal. My headlight modulator on my bike, for instance, pulses between 20% brightness and 100% brightness 240 times per minute, or 4 times a second. Its not a 'strobe', and it doesn't look anything like an emergency vehicle, and it doesn't cause target fixation, hence, its legal. If it really DID strobe, it would definetly be illegal.
Anyway, hope this helps! My personal recommendation would be to buy the unit that flashes 4 times in 3 seconds, then stays on solid, and install it on your third brake light only.
If you want to go with LED bulbs, be careful! There are a LOT of crappy ones on the market, that are actually DIMMER than regular bulbs. They should have 8 or more LEDS, preferably with some in a side-emitting stance, since your tail lights don't just shoot out the back, but also out the sides. Expect to pay more for something that is an actual improvement. I've seen a LOT of people buy cheap LED bulbs, and come away disappointed (and less safe) because of it. Yes, they turn on faster, but if they aren't any brighter, whats the point?
Read this page, which is a review of some LED bulbs for motorcycles, but they are usable in cars too, and there is a lot of good general information about that type of bulb:
http://www.webbikeworld.com/r2/led/led.htm
Enjoy!
Peace,
Sam