The Common App

http://www.commonapp.org/

The Coalition Application

http://www.coalitionforcollegeaccess.org/
(The University of Maryland is now a Coalition member)

Talk to College Students: 

The College T allows high schoolers to search for students who attend the colleges they’re considering, then connect with those students to ask them questions.

https://thecolleget.com/

Research Colleges

https://bigfuture.collegeboard.org

Essay writing

College Board:
https://bigfuture.collegeboard.org/get-in/essays/8-tips-for-crafting-your-best-college-essay

National Association of College Admission Counseling: http://www.nacacnet.org/studentinfo/articles/pages/top-ten-tips-for-writing-a-college-essay-.aspx

Suggestions from Jay Mathews, education reporter at The Washington Post:
https://www.washingtonpost.com/local/education/five-underutilized-ways-to-give-yourself-an-advantage-with-your-college-essay/2016/10/09/56d1c156-8dd6-11e6-875e-2c1bfe943b66_story.html

Essay examples

A recent essay that got its author into five Ivy League colleges
http://www.businessinsider.com/high-school-senior-who-got-into-5-ivy-league-schools-shares-her-admissions-essay-2016-4