The Maratac Split Pea lighter are, to my knowledge, the smallest lighters out there, and despite their mini size, they actually work really well.
The Split Peas work in a zippo-like fashion - liquid fuel with a wick and flint striker wheel - but have an O-ring and screw top, sealing them up air tight when closed. This keeps the fuel from evaporating out during storage - I've left these lighters in a drawer for six or so months and then tried 'em out - and, every time, they've lit up first time, just fine and dandy.
Because they run on liquid fuel, you can use lighter fluid, but, in a pinch, use other fluids as well. Not recommended, but they will work on most any flammable liquid. Obviously, the ability to refuel is a good thing - I have one of the slightly larger Peanut lighters in my bug out bag for just that reason.
Since they are metal, they are a bit heavy, but also pretty much bombproof, too. They'll hold up to riding on a keychain, no problemo.
The flat bottom allows them to stand on end and function as a candle. If you don't want the lighter portion, they body makes a nice pill or tinder carrier.
I've jammed a few extra flints and a spare o-ring into the bottom of mine, underneath the lighter portion - a mini repair kit always along for the ride!
The only downside is inherent with the tiny size - it can be a little hard to hold onto and light...cold hands would only make this worse! They are also a two handed deal - you've got to unscrew the top - and, if you are not careful, the top could easily be misplaced or lost.
The copper split pea looks to be currently unavailable, but the stainless steel and titanium lighters are up for grabs on the CountyComm website.