Badly chosen name just like DRM
zram in the kernel == lzo-compressed ram disk, usually used for swap. zcache as I understand it enables the compression of caches like the various filesystem caches. That it's on top of transcendental memory is just coincidence, the first versions weren't (IIRC).
I'm pleased with the results. I've been using this machine to run a Minecraft server while playing on it at the same time. Since using Bukkit (server mod), the RAM usage has been a bit heavier and it's started hitting swap, resulting in nasty slow downs. I tried it just now with zcache enabled and it didn't hit swap once.
Yeah, so they can do similar things but zram doesn't need the transcendental memory aspect (which is, IMHO, one of its --zcache-- big selling points).
I was thinking of this (http://lwn.net/Articles/397574/) and zram was mentioned in the comments (http://lwn.net/Articles/397894/).