Computer Science Fundamentals

Algorithms & Data Structures

VisuAlgo - visualgo.net
Visualize data structures and algorithms.

Algorithm Visualizer - algorithm-visualizer.org
Interactive algorithm visualizations.

Big-O Cheat Sheet - bigocheatsheet.com
Quick reference for time/space complexity.

Books

Introduction to Algorithms (CLRS) - The definitive algorithms textbook
Cracking the Coding Interview - Interview preparation Bible
Design Patterns - Gang of Four classic