There's very little data to speak of. Wikipedia references some site-meter type statistics at ENWorld, but there's a heckuva selection bias at ENWorld towards the current edition of D&D and against any other game. Not that they're not talked about, or can't be talked about, but that that simply isn't the hot place to talk about it.
Today's statistics on the Hot Roleplaying Games page suggest the OSR tag is used on 3.7% of the discussions had. That said; older games, or specific OSR games really should be added to that list, which brings it to 5.57%. Not bad, for a subset of gamers that probably don't really have much in common with ENWorld's M.O. to begin with!
Here are the games that I presume OSR themed gamers are using, to some degree or another:
- First off, keep in mind that WotC have released most of the actual older editions of the games, either as pdfs or as reprints. You can get 1e, 2e, B/X, OD&D, BECMI, and the RC at least this way. Presumably many players who would be drawn to retroclones don't actually need the retroclones if they can get the actual regular rules themselves; the retroclines then become more useful as a platform to enable other ancillary products like modules, settings, or customization options.
- It looks like Castles & Crusades is still kicking around. This was a curious hybrid; old school SRD before the OSR really started for realzies.
- I've mentioned specifically ACKS and LFP, but I don't know how much use these get. They seem to have gathered a lot of talk online, at least. Dungeon Crawl Classics probably belongs in that grouping as well. Neither are exactly retroclones, being rather evolutions of the retroclones in many ways.
- OSRIC and Labyrinth Lord were very early retroclones (of 1e and B/X, respectively) but I don't hear much about either anymore. Then again, I don't trawl through OSR blogs and whatnot, so maybe I'm just out of the loop.
- Swords & Wizardry, especially in its even more stripped down "White Box" version seems, from my limited perspective, to have quietly become the mechanical chassis of choice for much of what's going on in the OSR. Given that S&W is meant to be a retroclone of the original 1974 version of D&D, there's a kind of odd "circle of life" symmetry to this, if so.
What would I use if I were into the OSR? I'm not really, but I'd like to do a B/X retro-game for a time. I'd probably do B2: Keep on the Borderlands and then X1: The Isle of Dread specifically and then can the game. I'd also like to do a more old-school style game for a time as an experiment, or just for fun; a good old-fashioned hexcrawl. But I probably wouldn't actually use old rules for it; I'd probably use an m20 variant. Purest Essence seems like the most D&Dish of the bunch without getting specifically into more complicated or esoteric variants, but Microlite74: Standard Edition might be a good choice too.