bzip2 is a freely available, patent free (see below), high-quality data compressor. It typically compresses files to within 10% to 15% of the best available techniques (the PPM family of statistical compressors), whilst being around twice as fast at compression and six times faster at decompression. 

http://sources.redhat.com/bzip2/

There are Tcl bindings available for it [bztcl] . It would be a great addition to have a Tcl VFS backend available