Recent work has been searching for general principles of organization and evolution of natural and artificial systems changing through local rules based on reuse of previously existing substructures. Such a process of "tinkering" makes a big difference (at least in principle) when comparing biological structures and man-made artifacts. As pointed out by the French biologist François Jacob, the engineer is able to foresee the future use of the artifact (i.e. it acts as a designer) whereas evolution does not. The first can ignore previous designs, whereas the second is based on changes taking place by using available structures. In spite of its apparent drawbacks, tinkering has been able to generate most complex structures observable in the real world (including some in the technological world). Very often, the resulting structures share common principles of organization, suggesting that convergent evolution towards a limited number of basic plans is inevitable. How innovations emerge through evolution is one of the key problems in complexity. Recent work on evolved complex networks suggests that tinkering is a main driving force shaping complex systems and that several desirable properties, including modularity, might emerge for free under tinkered evolution.