The latest QuantLib release is version 1.6.1 (August 3rd, 2015).
Source packages for all platforms are available from the SourceForge download page in a number of different formats. Compiling QuantLib requires a working Boost installation; Boost 1.39 or later is needed. Advanced users may want to configure the library with non-default settings.
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. Eager Debian users not wanting to wait for the library to be packaged may acquire QuantLib from the SourceForge link above.