Ok, so this question gets asked alot and the most current resource for this information that I could find is 6 years old, so I decided I was going to do some research and update it.
Below you will find a list of publishers currently accepting game submissions online, a list of those only accepting submissions at cons/events, a list of those that are co-producing only or there is some other arrangement, and a list of those with conditions to change in the future.
I also listed what the publishers are looking for or not looking for if it was specified.
This is what I have found. I am sure there are others. Please add in the comments and I will add them to the list.
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ACCEPTING GAME SUBMISSIONS
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Academy Games
http://academygames.com/contact-us/submission-guidlines
Amigo Spiele
http://www.amigo-spiele.de/index.php?option=com_content&view=article&id=...
No interest in: 2 player only games, sport simulations, economic simulations, role playing games, trading card games, dice games run with event fields, games with huge wood content. Bad odds with purely abstract games and extremely complex games.
Cambridge Game Factory
http://www.cambridgegames.com/index.php?page=submissions
Focused on "Euro" games, but would certainly consider simple war games that could be marketed to a mainstream audience.
Cocktail Games
http://cocktailgames.com/en/cocktailgames/faq
Looking for: games involving communication, atmosphere, tactics, for kids, adults, aliens. 10 to 30 minutes. What can we put in a box? Our little boxes can hold no more than fifty square cards (for the Pocket range), and no more than a hundred rectangular cards (for the Slim range). However, some cards can be replaced by cardboard tokens, plastic pawns, etc. So please avoid sending us board games.
Cranio Creations
http://www.craniocreations.com/en/publish.php
Crash Games
http://crashgamesaz.com/contact/
Accepting all types. Preference towards micro-games.
DaVinci Games
http://www.dvgiochi.com/contatti.cfm#24
Interested in games with simple rules (1-4 pages) and limited duration (15-45 minutes) for 3 or more players. Looking for high interaction for many players and party games or original games that are quick and cheerful.
Escape Velocity Games
http://www.escapevelocitygames.com/game-submission/
Looking for small projects (standard components and small boxes), not interested in abstract, sports, puzzle, word, educational, or traditional RPG (pen-and-paper) games.
Flatlined Games
http://www.flatlinedgames.com/submissions
Game Salute Games
http://gamesalute.com/game-submissions
Uses Kickstarter exclusively.
Gamelyn Games
http://gamelyngames.com/cms/submit
Currently accepting submissions for: Micro-Games plays in 10-20 mins, Small Board Games plays in 15-30 mins.
Hans Im Gluck
http://www.hans-im-glueck.de/english/
Indie Board and Cards
http://www.indieboardsandcards.com/publishers.php
Doesn't look at abstracts, wargames, 18xx, two player only, CCGs/LCGs. Prefer games that are not listed on BGG (yet!) and haven’t been web (PnP) published.
Le Scorpion Masque
http://www.scorpionmasque.com/en/contact
Letheia Games
http://www.letheia-games.com/propos/
Libellud
http://en.libellud.com/contact-and-location
Game must be original and aimed at a family audience. Games should not last longer than an hour. Not interested in abstract games, letter-based games or derivatives of classic games such as Monopoly, Scrabble, etc.
MAGE Company
http://www.magecompany.com/submissions.html
Mayday Games
http://maydaygames.com/publishing.html
Rulebook and a video demonstration of the game being played is required.
Paille Editions
http://www.paille-editions.com/auteurs-illustrateurs
Repos Productions
http://rprod.com/index.php?page=contact-2
Smirk & Dagger
http://www.smirkanddagger.com/contactus.htm
Looking for: Nasty, gotcha games.
Van Ryder Games
http://www.vanrydergames.com/game-submissions/
Not currently looking at: war games, heavy Euro games, Zombie games
Victory Point Games
http://www.victorypointgames.com/submissions/overview.html
What's your Game?
http://www.whatsyourgame.eu/yourideas
Mainly seek boardgames and cardgames both for families and expert players use, but prefer not to limit the creativity of authors.
Wishing Tree Games
http://www.wishingtreegames.com/submit-a-game.html
At this time, we are specifically looking for micro-games.
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ACCEPTING ONLY AT CONS/EVENTS
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Level 99 Games
Mayfair Games
Minion Games
Tasty Minstrel Games
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CO-PRODUCING OR OTHER ARRANGEMENTS
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Nestorbooster
http://www.nestorbooster.com/#fueled
nestorbooster was a crowdfunding platform, but it has now mutated into an authorfunding platform where authors fuel their own games by buying copies in advance.
Eagle/Gryphon Games
http://images.eagle-gryphon.com/Submission_guidelines.pdf
Requirements may include: assembly of 6-8 playable prototypes with a complete and well-presented set of rules; oversight of a Kickstarter project for the game (under our auspices and with direction from us); and pay a portion of the production costs.
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CONDITIONS TO CHANGE
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Gigamic Games
http://en.gigamic.com/faq
Only review proposals between March and September.
Grey Gnome Games
http://greygnome.com/about-us/
Looking to add games from other designers in the future.
Stonemaier Games
http://stonemaiergames.com/submission-guidelines/
Most interested in finding a a large-group game in the style of The Resistance, Coup, and For Sale (ideally 2-10 players, but we’re flexible on that) that’s easy to teach, offers a lot of group dynamics, doesn’t have player elimination, and is ideally set in the world of Euphoria. Also interested in a cooperative game, possibly set in the world of Euphoria, but other themes are fine too.
Submissions are temporarily closed, but will reopen in April 2015.
Yeah I hadn't heard of half of them either before I looked them up!