Research
Pubspace is the latest revolutionary initiative by NASA to offer all its Research papers for public access for free. Also, you can submit your research papers for publication over Pubspace.
NASA started offering few of the space program software and code for public. Its a great source for engineering research.
Google Scholar is a great place to find a huge database of technical research papers.
Microsoft Academic offers millions of technical research documents for public access.
IEEE Xplore digital library offers millions of technical research documents for public access.
MIT Open Courseware digital library offers vast number of technical research documents for public access.