Not sure what your getting at but I think the Sound Lab A-1's and Apogee Scentilla both have an .5 ohm impedance at some point in their operating range.
Very few amplifiers are capable of properly driving such a low impedence speaker.
I too am not sure what you are really asking. One wouldn't ask what speaker is able to..., but either what speaker does, or what amplifier is able to drive such a speaker.
Adding to the above list, the Cello's biggest speakers operated at about one ohm.
Sound Lab A-1's go down to around or slightly below 2 ohms in the very high treble, to the best of my knowlege.
I used to use Strathearn ribbon drivers in homebrew speakers, and they were a .55 ohm load.
Amplifiers that can drive such a load include the Electron Kinetics Eagle 2 (which is what I used on the Strathearns) as well as the InnerSound ESL amp (stable into a .33 ohm load). I think the Claytons are also stable into .5 ohm loads (Disclaimer - I peddle these two lines, which is why I'm familiar with 'em). I'm sure there are many others.
It's too bad the amp-killer early Apogees and early CLS's aren't still being made, as these days there are relatively affordable amps that can drive 'em.
I suggest the question should be what amplifiers are capable of driving loads of 0.5 ohms, not what speakers are able to (huh?) go down to 0.5 ohms. By the way, AFAIK Sound Labs don't go that low, more like in the ballpark of 2 ohms or perhaps a bit lower in the high treble, where there is little energy anyway. But it's true that Apogee Scintillas go that low, and as Avguygeorge mentions, Martin Logan CLS I's dip to 1 ohm or less. There haven't been many amps able to drive Scintillas well, although a small handful are capable of driving a dead short and handle them just fine.
That reminds me of a story a friend was telling me recently about the infamous John Iverson who used to carry his little Eagle 2 amp around to dealers and tell 'em how it could drive a dead short - and as part of his impressive demonstration he'd prove it by disconnecting the speaker cables from the speakers and shorting them together while music was playing through them - and the darned amp would weld the speaker cable spades together! Utterly scary yet based upon what I've heard about Iverson, I have little reason to doubt it's true. I knew a fellow quite a few years ago who owned an Eagle 7a amp (the big one), and it was a sweet sounding brute!
Hey,Duke Thanks again and again for the gems of wisdome you pass our way. 'Been reading your posts for years; never said thank you before. Not to late huh?? -----
I actually own an Eagle. I did the upgrade to 4 a couple of years ago. Here's something that blows my mind--well sort of. I use the amp to drive my center speaker;the Von Sweikiert lcr 35. When I play the THX test on some dvds;then you have the sub. tones.---I have no sub---BUT, I get a low rumbling bass tone from the center speaker.Oh, it's a clean rumble. I know the dif.Must be the amp??
This review of wolcott amps driving A-1's was the source of my incorrect info. about the A-1's impedance. He states the impedance at .8 ohm and not .5 ohm
As this is a review of the wolcott's and not the speakers, the reviewer my be wrong about the A-1's impedance.
Of course the low impedance of the Soundlab is in the extream highs so is nothing like the Scintilla. It's the high impedance of the Soundlabs that kill the big SS monster amps.