Applications of Game theory in computer science?
One of the most famous examples of game theory in computer science is Yao's minimax principle.  Let $X$ be a set of inputs for some problem, and let $A$ be a set of (deterministic) algorithms for that problem.  Yao's principle states that
$$
\max_{x\in X} \operatorname{E}\limits_{a\in A} \left[T(a,x)\right]
\ge
\min_{a\in A} \operatorname{E}\limits_{x\in X} \left[T(a,x)\right] ,
$$
where the expectations on the left and right are taken with respect to any desired probability distribution over algorithms and inputs, respectively.

For example: Any deterministic comparison-based sorting algorithm requires $\Omega(n\log n)$ time on average to sort an array permuted uniformly at random.  (Proof: In any binary tree with $N$ leaves, at least half the leaves have depth at least $(\lg N)/2$. $\square$)  So Yao's principle implies that the worst-case expected running time of any randomized comparison-based sorting algorithm is also $\Omega(n\log n)$.

Yao's minmax principle follow easily from von Neumann's minimax theorem for two-player zero-sum games, where one player provides the input and the other provides the algorithm.