The following instructions are deprecated for QGIS 3.
Proprietary (closed) formats are a pain in the a_s for both users and developers of Open Source software. The ECW format is a common format for imagery in GIS software, but unfortunately to read/write it you will need closed (and paid) libraries. Long story short: the latest version of this libraries (that allow to read, but not write ECWs) are free of charge but are not redistributable, this means that cannot be shipped together with software packages like QGIS. The good news is that are still available old libraries from the times when the ECW format was owned (and developed) by a company that used a more permissive license. So:
c:\program files\gdal” and “c:\program files\gdal\gdalplugins and pick two
On Ubuntu Linux:
Follow the instructions here not before
after having added the Ubuntugis repository
https://launchpad.net/~ubuntugis/+archive/ubuntugis-unstable and installed the