.7z is the native format of the 7-zip archiver (http://www.7-zip.org/)

7-Zip is a file archiver with high compression ratio.
7-Zip is free software distributed under the GNU LGPL.

Exactly this is the reason why we decided to provide some
files also in 7z format. They are nearly half the
volume of the corresponding .zip file. This helps
especialy modem users to save time but it also saves
the 45MBit user some time and sf.net some load.