PAQ is a series of archivers that achieve very high compression rates at the expense of speed and memory.
To compress: paq8o4 -5 archive [files_or_folders...] (creates archive.paq8o4)
To decompress: paq8o4 -d archive.paq8o4 [output_folder]
The -5 option is the default. It requires 233 MB of memory for compression and same for decompression. Options may range from -1 (35 MB) to -8 (1643 MB). More memory usually means better compression. In the Windows version you can also compress or decompress by putting paq8o4.exe on the desktop and dropping files or folders on it. Compressed files/folders have a .paq8o4 extension. The input is not deleted.
NOTE: Files can only be decompressed with the same version of PAQ that they were compressed with. However, links to all versions can be found here. Decompression requires the same memory and time as compression. PAQ is still experimental, so you should expect frequent updates.
There are three files:
paq8o64 - AMD64 version,
paq8osse - SSE2 version,
paq8o - Generic MMX version.