I have worn an Arc-AAA, and more recently, a Peak Matterhorn aluminum lug style single LED around my neck on a piece of nylon string for the past 4 years or more. Neither light is inexpensive at around $35 new, but it's always there when you need it, runs on AAA batteries, has never turned on accidently, and is virtually impossible to lose.
I greatly prefer the Peak's 'snow' LED to the rather ugly beam produced by the Arc-AAA, but either light is very usefull.
I like to use lithium AAA cells, due to their lighter weight. They cost about $2/ea., but as this kind of light gets intermittant use, a single AAA cells lasts me for months.
You can likely find a nice used Arc-AAA on the B/S/T section for a lot less than a new one. The newest generation Arc-AAA Premium model puts out more light, so there are a lot of the earlier versions not in use. I know I have 4 or 5, myself. Put an ad in the WTB, and see what turns up.
I know at one time new Arc-AAA's were selling for less than $20.
Hmm... as an afterthought, if you want a bright light in a small package, the Peak Matterhorn 3 LED is the same size as the single LED model; that's what I carry on my keychain. It's a high, rather than ultra power, but it's is amazinly bright with a fairly decent runtime. Fenix makes 2 models with Luxeon type emittors, but while they are very bright, runtime is as short as an hour or 2.