The following instructions are deprecated for QGIS 3.
The support is enabled by default on Windows, and probably no longer necessary on other Operating Systems.
For all your needs on GIS data management, do not hesitate contacting us.
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:
On Windows:
http://www.gisinternals.com/sdk/Download.aspx?file=release-1600-gdal-1-9-2-mapserver-6-2-0\gdal-19-1600-ecw.msic:\program files\gdal” and “c:\program files\gdal\gdalplugins and pick two .dlls libecwj2.dll and gdal_ECW_JP2ECW.dlllibecwj2.dll into c:\OSGeo4W\bingdal_ECW_JP2ECW.dll into c:\OSGeo4W\bin\gdalplugins\1.9On Ubuntu Linux:
Follow the instructions here not before
after having added the Ubuntugis repository
https://launchpad.net/~ubuntugis/+archive/ubuntugis-unstable and installed the
libgdal-ecw package.