Mobile Fidelity Sound Labs used to remaster vinyl records at half speed. This considerably improved the quality, and the records retailled for around £25 - they're well worth tracking down, as they can be obtained for more or less the same amount, depending on the album (e.g. Sgt Pepper will go for more like £100, while "Crime of the Century" would go for £5 or so).
Since MFSL are doing CDs, then half-speed mastering is pointless, and 24-bit remastering only works so far on a format that is essentially 16-bit, so they've come up with ways to improve the quality of the CD itself. I haven't heard one, but I'd suspect that DVD-audio is superior, audibly (even if the media doesn't last as long!).
http://www.mofi.com/