These don’t all really seem to necessarily be “equations” but they are all extremely useful things you should know. The article contains some details on the following:

• Binomial Coefficient
• Demorgan's Laws
• Eigenvector and Eigenvalue
• Pumping Lemma for Regular Languages
• Information Entropy
• Bayes' Theorem
• Fermat's Little Theorem
• Natural Join
• The Fixed-Point (Y) Combinator
• O(N)
• Euler's Identity

Eleven Equations True Computer Science Geeks Should (at Least Pretend to) Know (via elegantcoding.com)