Impact of Grothendieck's program on TCS
Grothendieck's inequality, from his days in functional analysis, was initially proved to relate fundamental norms on tensor product spaces. Grothendieck called the inequality "the fundamental theorem of the metric theory of tensor product spaces", and published it in a now famous paper in 1958, in French, in a limited circulation Brazilian journal. The paper was largely ignored for 15 years, until it was rediscovered by Lindenstrauss and Pelczynski (after Grothendieck had left functional analysis). They gave many reformulations of the paper's main results, related it to research on absolutely summing operators and factorization norms, and observed that Grothendieck had solved "open" problems which had been raised after the paper was published. Pisier gives a very detailed account of the inequality, its variants, and its tremendous influence on functional analysis in his survey. 

Grothendieck's inequality is very naturally expressed in the language of combinatorial optimization and approximation algorithms. It says that the non-convex, NP-hard optimization problem 
$$
\max\{x^TAy: x \in \{-1, 1\}^m, y \in \{-1, 1\}^n\}
$$
is approximated up to a fixed constant by its semidefinite relaxation
$$
\max\{\sum_{i,j}{a_{ij}\langle u_i, v_j\rangle}: u_1, \ldots, u_m, v_1, \ldots, v_n \in \mathbb{S}^{n+m-1}\},
$$
where $\mathbb{S}^{n+m-1}$ is the unit sphere in $\mathbb{R}^{n+m}$. Proofs of the inequality give "rounding algorithms", and in fact the Goemans-Williamson random hyperplane rounding does the job (but gives a suboptimal constant). However, Grothendieck's inequality is interesting because the analysis of the rounding algorithm has to be "global", i.e. look at all terms of the objective function together. 

Having said this, it should not be surprising that Grothendiecks's inequality has found a second (third? fourth?) life in computer science. Khot and Naor survey its multiple applications and connections to combinatorial optimization. 

The story does not end there. The inequality is related to Bell inequality violations in quantum mechanics (see Pisier's paper), has been used by Linial and Shraibman in work on communication complexity, and even turned out useful in work on private data analysis (shameless plug).