The latest QuantLib release is version 1.21 (January 20th, 2021).
Source packages for all platforms are available from GitHub in both zip and tar.gz format for QuantLib and QuantLib-SWIG. Compiling QuantLib requires a working Boost installation; Boost 1.48 or later is required, but a more recent version is suggested. Advanced users may want to configure the library with non-default settings.
There are no official binary packages, but volunteers provide them for a number of platforms. Mac OS X users can use Homebrew. Debian users can find the latest version of the different QuantLib packages in the "unstable" (a.k.a. "Sid") distribution; see the Debian site for a list of available packages. Ubuntu users can use Dirk Eddelbuettel's PPA. Other Linux distributions might also package the library; check your sources. Users not wanting to wait for the library to be packaged may acquire QuantLib from the download link above.