I.D bands

+1 for RoadID

this new one on me loks interesting as well - http://www.onelifeid.com/

another for Road ID

Me and Mrs S have got the cram alert:

http://www.cram-alert.co.uk/

but others are available e.g.

http://www.iceid.co.uk/content/id-for-runners.htm

Ice Tags are another option. Just a simple plastic tag with your info on it and if you're a Parkrunner, you can also get your barcode printed on the back as well.

I have an Ice id, very nice quality, Oh i should ad I have the Iceid Pro

I got a road if after being knocked off my bike by a hit and run driver on a deserted road last year ( bit shook up but fine ).

I never run or cycle without it, just hope it is never needed. Great service, arrived from the states in less than a week.

Road ID, it arrived very quickly considering it's a US based company!

Got aluminium dog tag with name, d.o.b, blood type, ice info engraved on it. Cheap as chips.

+1 For Road ID .  Great service, easy to use website, many varieties

The only (slight) issue I have with these, is the permanance of them.

If you change (mobile) phone numbers, for example, you have to replace the engraved/printed ones

I know mines not as immediately obvious, but I wear a SOS Talisman

http://www.sostalisman.co.uk/brochure.html  (standard one, top left)

The paper strip can be updated (comes in packs of 5?)
Granted, it's not as tough as dog-tags, but possibly an easier proposition to wear '24/7'??? (for normal 'day-2-day' activities, rather than just training/racing??)

gosport ID 

sign up to this free site http://earndit.com and link it to your garmin/runkeeper etc

once you have done about 3 runs you will have enough points to get a free ID band from the vouchers you earn,
http://earndit.com/reward/view/id/97/10-gift-card-to-go-sport-id

the rest of the vouchers i found worthless, but it was worth signing up for that, cost me £5 postage thats all.

Another for Road ID

Very quick to send, normally within a week.