What it says. I'm only doing this this out of boredom. This is mostly older companies or companies known for releasing older titles
Synapse: These guys are great. They've released all kinds of classic titles in the past ("Street Trash", "Evil Dead Trap") and are now going into Blu-Ray with releases of old Hammer titles, the final "Coffin Joe" movie "Embodiment of Evil", and the slasher "The Dorm that Dripped Blood", as well as releasing old titles from the likes of Panik House and Elite Entertainment. Speaking of Elite...
Elite Entertainment: They've fallen on hard times. They aren't dead (they have two new titles coming out in June), but they seemed to lost the rights most of the older material they released ("Re-Animator", "I Spit on Your Grave" and "House on Sorority Row") to other companies, with only a few still in print, or from the last decade.
Code Red: Kind of a Godsend for fans of largely forgotten horror and exploitation, Code Red have released a few classics ("Messiah of Evil" and "Soul Survivor"), some oddities ("The Visitor", "Boardinghouse") and some real duds ("Night of the Dribbler.") Still, if genre fair like "The Carrier" and "Pets" are your cup of tea, then these guys are for you.
Media Blasters: Sadly, they don't seem to be releasing any new releases of older titles, and seem to be more about Anime and Blu-Ray releases of their back catalog at the moment. Still, they've released some well loved titles in the past, so hey, check those out.
Anchor Bay: They're the guys who got a few titles from Elite ("The Evil Dead", "Re-Animator" and "I Spit on Your Grave"), but since being purchased by Starz Media Group, they're more about releasing their own titles, or Sci-Fi Channel movies. Oh, and yes, they're still re-releasing "The Evil Dead" on a regular basis.
Blue Underground: I love these guys. They've been releasing special editions and Blu-Rays of old titles Anchor Bay used to release, as well as so much more. They've got special editions of "Deep Red", "Torso" and "Zombi 2" on the way, which makes me a happy camper.
Dark Sky: They've released older titles in the past (including a superior release of "Horror Hospital"), but this studio seems to be releasing newer titles with the help of IFC and Magnolia.
Unearthed: Admitting, I'm not a huge fan (a lot of mock snuff films and shot on video movies), but they have released special editions of "Rock and Rule" (love that movie) and "Frankenhooker."
Troma: Still kicking around, though I wish they'd learn how to do better transfers on some of their releases (especially "Mad Dog Morgan."
Severin: They're mostly known for releasing "Birdemic: Shock and Terror", but they've also released the classic "Santa Sangre", Paul W.S. Anderson's "Shopping", the cult faves "Psychomania" and "Hardware", Lucio Fulci's "The Psychis", and so much more. These guys know what they're doing.
Redemption USA: They've released plenty of erotica, but I'll always know them as the studio that released several titles from the late Jean Rollin. Also, not made by him, but I gotta see "I Bought a Vampire Motorcycle."
Mondo Macarbo: They've released plenty of offbeat obscurities (such as "The Sadist With White Teeth"), but they're probably mostly known for giving the world Bollywood horror titles and really bizarre horror and exploitation from the Philippines. I have nothing else to add other than the fact that you must see "Lady Terminator."
And those are just a few off the top of my head.