Most useful Posts
Academic Search Engines
JURN, One can dig through a couple of million articles.
DOAJ, A rather small database for academic books, only. They also have one for articles.
RefSeek, A well-equipped search engine with an excellent design. Similar to Google with the slight difference that you can peek into documents without having to open them. Saves a lot of time if you want to perform a quality check or discover how often your keyword can be found inside.
Make sure to click "Documents" instead of "Web" on the right side above the search bar. This ensures that you only search for academic research papers.