I spent a fair amount of hours today downloading all environment packs from humus.name. I command line processed each and every one of them to a 1024px (at mip lvl 0), pre-blurred the mips (by mip initial filter angle of 3) and saved them as uncompressed A8R8G8B8 DDSs. I also compiled a preview-sheet with a thumbnail of each cube (the Z+ face) to easier find something suiting by general color/mood/lighting.
These cater directly to my Maya CGFX shaders as they have pre-blurred mips for altering reflection and ambient blur amount, but I'm fairly certain that others might find this useful as well.
As there are 89 cube maps in this pack and each of them is 32MB, even when compressed it's all together ~900MB. I decided not to host this on my site and instead try a "free download site". We'll see how it holds up.
EDIT: The "bahnhof" links are through my ISP which allows me to host my own website. Apparently no traffic cap when I phoned them about it. We'll see. Use the 4shared if the bahnhof's don't work or are slow. There's also a torrent link you can use as well(thanks jramauri).