A great website that strives to provide high-quality materials taught by preeminent professors at MIT. The courses are available for free for everyone. The website currently has 2150 courses, and you can find many courses for both undergraduate and graduate students in a variety of fields including Electrical Engineering and Mathematics (link here).
It is a non-profit educational website created by Salman Khan, a graduate of MIT and Harvard Business School. It provides great short video lectures for free for a variety of topics. Such topics include mathematics, physics, etc. Its Youtube channel contain more than 27,000 videos! The website can be found here, and the YouTube channel is available here.
A digital library that provides access to scientific and technical content published by the IEEE and its publishing partners. There is no doubt that this library is the number one library in the world for graduate students and professors in several fields spanning Electrical Engineering, computer engineering, etc. (link here- You need a subscription to access most of the articles).
Each month, the IEEE Communications provides a selection of free tutorials on selected topics in communications. These tutorials are relatively long (2.5-5hours), yet they are really informative and useful. I always recommend to attend seminars or tutorials even in topics that are not very related to your research area because interacting with people of different mind-sets, and even research areas, can enlighten your thinking and improve your aptitude (link here).
A great resource to simply and broadly search for scientific articles and journals, and more importantly, a great website to follow up with recent publications of both professors and students (link here).