Crossref is an official digital Object Identifier (DOI) Registration Agency of the International DOI Foundation. It is run by Publishers International Linking Association Inc.
Google Scholar is a freely accessible web search engine that indexes the full text or metadata of the scholarly literature across an array of publishing formats and disciplines.
Directory of Research Journal Indexing
DRJI provides ready access to the education literature to support the use of educational research and information to improve practice in learning, teaching educational decision-making and research.
Scientific Indexing Services
SIS offering database services to researcher. It's mainly: citation indexing, analysis and maintains citation databases covering thousands of academic journals, books, proceedings and any approved documents SIS maintains academic database services to researchers, journal editors and publishers.
Ulrich's is an index that lists over 300,000 periodicals and journals by title, and provides information on the publisher, indexing and subjects. It can be used to determine whether a journal is peer reviewed (refereed).