Aggregators refer to the online platform that links you to a variety of courses being offered by different institutions and platforms around the world. You will find courses from prestigious universities such as Harvard, Stanford,John Hopkins and Duke University among others.
Below are some of the search engines you will find on the web.
 
Class central is widely used and has very many courses being offered. You can access courses from over six hundred learning institutions in Class Central. To view the list of institutions visit the List of Universities.  
 
MOOC List is a compilation of  MOOCs in offering across a diverse range topics by a variety of prestigious universities. 
CourseSites MOOC Catalog is another web page that collates available MOOCs. It is a good page to keep you updated on the latest MOOCs offering.
 
Course Buffet, is an online catalog that allows to find and compare a lot of free online courses from over 250 universities.
Edukart is is an Indian based online education platform providing information on MOOCs globally.
 
The sites named above are just but a few aggregator sites therefore, you will come across more similar platforms on the web.