I have medicalert, you can get various types and styles, from dog tags and bracelets to watches etc...
The information engraved on the back is the critical information, my large tag has 8 lines (ok some are 1 or 2 words) on them and there is a unique serial number and phone number that a doctor or hospital can call to get a full list of your medical conditions, and also all your current medication and your next of kin.
In addition you get a printed card with all the information to put in your wallet.
Its an annual subscription and is a worldwide recognised service. So far I've not has cause to use it, but I've had the tag for 7 years now.
With all of these tags, remember you need to wear them 24/7 and as such if you have acidic skin then plated tags will corrode quite quickly. It might therefore be worth investing in a solid gold tag day one, I went through three SOS Talismans (silver and gold plated) in a couple of years through corrosion, as I said the solid gold one has lasted 7 years and will last a good many more. Shame really as I hate wearing jewellery!