Make Room! Make Room! by Harry Harrison, filmed as Soylent Green in 1973
Was The Blob based on a book?
Invasion of the Body Snatchers by Jack Finney, first published in 1955–there are 3 motion pictures.
Ooh - you beat me to Laumer! I recently discovered him myself, thanks to The Baen Free Library - they’ve got a Retief short story collection, another ss collection called Keith Laumer: The Lighter Side and Galactic Odyssey.
Jack McDevitt is a really good, not-real-well-known author. “The Engines of God” is terrific. About an alien intelligence out to destroy all other forms of intelligence in the universe. “Eternity Road” is a great post-apocalypse novel.
I’ll second (or third) Piper. Liked the Fuzzy books, as well as the ones written by other authors.
I also enjoyed the Matador series by Steve Perry. Although I don’t think he’s an obscure author, the series itself isn’t well know. First book in the series is The 97th Step. Good read.
I enjoyed Death World as well as it’s two sequals, by Harry Harrison.
I also enjoyed the *Fire Dancer * books by Ann Maxwel. Those are a bit harder to find, but I did like the stories.
Holy crap! What a response! This will keep me busy for awhile. Thanks, Dopers.
Eric Frank Russell: The Sentinels of Space, Sinister Barrier, Wasp, Three to Conquer. Trust me, he is the most underrated SF author of all.
A writer named Louis Charbonneau wrote some decent books, but I can’t remember titles off the top of my head.
John T. Sladek’s Mechasm
William Sloan’s The Edge of Running Water. He also wrote To Walk the Night, which I have, but have not yet read.
John Wyndham’s Out of the Deeps, also known as Kraken, and Rebirth.
I second Zenster’s remarks about Hodgeson. One of my dungeons is based on The House on the Borderlands. I also agree with Jeffwc’s remarks about Brown’s Martians, Go Home, although I don’t think Brown can be considered obscure.
Funny you should mention Wyndham: I am about halfway through Triffids right now. I really like his writing style. I guess I haven’t read many British authors in the past, and it seems to be a breath of fresh air.
I preferred Lem’s Cyberiad better. Freak’n hilarious man. 
Peter Watts Starfish is an amazing novel… I recommend it highly.
Dunno how obscure they are, but I really liked The Eyes of Light and Darkness by Ivan Cat with Darren Savari.
I couldn’t find a link for her, so I’d say Clare Bell, with her series starting with Ratha’s Creature counts.
Bruce Sterling is a well-known author, but his early works don’t get the attention they deserve. Check out Involution Ocean – closer to traditional SF than his later cyberpunk output, but still showing a distinctive style and imagination.
Avram Davidson (1923-1993) deserves even more attention than he gets. He wrote more fantasy (and mystery – he was one of the several authors who wrote as “Ellery Queen”) than SF, but his style has been widely praised, and if you read even one of his stories you will understand why. See the Avram Davidson Website, http://www.avramdavidson.org/.