Magic Numbers: Integers by Josh Poley from Microsoft Corporation - I found this article very useful, and this could help testers as a cheat code for developing input validation tests with integers.
Test Heuristics Cheat Sheet by Elisabeth Hendrickson - This is one of the best cheat sheat that you could find for testing.
The Power of Heuristics and Mnemonics by Parimala Shankaraiah - In this post Pari has compiled mnemonics to remember heuristics and oracles from various creators.Heuristic Risk-Based Testing by James Bach - A must read for all testers.
Heuristic Test Strategy Model by James Bach - Again, a must read.
Ten Usability Heuristics with examples by Jakob Nielsen & Theresa Neil - I found the heuristics with examples very helpful.
Great Resources by Huib Schoots