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
LES: There must be some mistake above. While bzip2 is indeed the strongest compression format and produces the smallest files, I've also found it to be one of the slowest formats. It's usually twice as *slow* than other formats, not twice as fast.