What are OERs
Open Educational Resources are free learning resources (typically online). The most famous being Khan Academy. 'Open' does not mean you have access to the sources of the content (in other words you can only see the final result) but you have access to the content itself free of charge. Most OERs are either funded by large organizations (such as the Gates or Hewlett Packard foundations or Google in the case of Khan Academy) or are self-funded and have a business model where they get support via donations of add-on services.
The challenge with OERs is that in some cases, you 'get what you pay for'
Indeed, quality can be spotty. Not out of bad faith of the developers, but that digital content is expensive to develop, host and maintain.
From this list we will excluded non-curated sources. For example, YouTube is probably the largest container of OERs. Some developed by teachers, some students helping students, etc. We also exclude Twitter and Facebook, which in themselves are not holders, but contain thanks to the dynamicity of teachers and students, links to OERs, big and small
The most common & respected ones
Khan Academy: Best known for its videos, they have built some learning trees and quizzes. Definitely worth the time when you start looking. They cover a lot more than just K12, going to college courses and professional skills. What is nice about Khan Academy is that they released the source code of their website, so you can build your own!
CK12: A very nicely organized and developed repository of tagged content and learning objects. If you are looking for structure, this is your first stop
SAS Curriculum Pathways: A little-known but truly amazing content repository. If it was not for OERs being a crowded space and that their parent company SAS is not known by the public, this resource would have been one of the top K12 resources
OERCommons: A very well-organized library of great content. Its core strength is the interface which allows for groups
OpenEd: One of the largest repository of assessments
Gooru: Once a high-flyer and darling of California schools, seems to be low on funding and content is getting outdated. Regardless, worthy of respect
Future of OERs
This is a dynamic space. However, the top 4 in this list have been around for a while and only getting better. These 4 are definitely the place to start building your own library of content. At this point, unless you have a great reason (and know what you are doing) you may want to steer clear of smaller players who are having a hard time keeping up with the competition, low funding and the dynamic nature of this space. Many of them are around as an outlet for foundations to spend their money and impress their boards.
As the larger publishers play catch up in the digital space, amazing content is at your fingertips and for free.
The main challenge is that teachers are now in charge of content
Not that it's a bad thing, it means more work for teachers. The days where you are given a textbook and told to follow the pages are gone.