.7z files can be opened with 7-Zip (
http://www.7-zip.org/)
Its pretty much the same thing as WinZip...
Winrar can also open .7z
IMO small programs like PCSX2 should be compressed to .zip ....I mean why bother using something else when you know that EVERYONE can open .zip while you actually need to get either Winrar or 7-Zip (and maybe the latest WinZip but since i only use Winrar i dont know...) to open .7z files