Research use: Tigress is free to use for research purposes.
Commercial use: If, after trying out Tigress, you decide to use it in a commercial setting you will need a license from the University of Arizona. Contact us and we will put you in touch with the university's licensing department.
Binary distributions: Available for 3 platforms: Linux x86/64, Darwin x86/64, and Linux ArmV7 (Raspberry Pi 3). Note that it is possible to cross-compile; i.e. you can run Tigress on a Linux x86/64 system and generate output that runs on a 32-bit system.
Source distributions: The FAQ explains why Tigress is not open source. We are happy to share the code with you if you are a researcher at a university or a research lab and we have done so many times in the past. Download the encrypted sources below and contact us to request the decryption key. If you're a student, please ask your professor/advisor to make the request for you. Keep in mind that in order for the source to make any sense you will have to be fluent in both OCaml and CIL. Generally speaking, it is much easier to understand what Tigress does by examining input/output-pairs (remember, Tigress is C source to source, so this is easy to do) than to try to read the Tigress source code.
Versions: The current Tigress version is 3.1. Version 3.0 can be downloaded here: binary, source. Pre-version-3 versions can be downloaded here.