We systematically study the optical spectra of ZnO grown by atomic-layer deposition as a function of Al (and Ti) doping concentration. The spectra measured on films are well described by fits using a Drude free-electron model. The derived plasma frequencies are consistent with the expected amount of doping and can be continuously and controllably tuned from small values to about 400 THz. The losses (damping) are also quantified. In addition, we achieve smooth conformal coatings of three-dimensional polymer templates made by direct laser writing. Altogether, Al:ZnO appears as an attractive "tunable metal" for three-dimensional infrared metamaterials or transformation-optics architectures.