Electronic music, from the avant-garde of Alvin Lucier to the minimalism of Steve Reich to the various vinyl and laptop specialists in international clubbing, has its growing share of classics. One surefire entry on the pop-rave side of things is “Rez” by the band Underworld, a trenchantly pulsing rising tide of percussive synth riffs that’s a frequent highlight of their live shows. Underworld has been posting some free MP3s, and some web-only commercial downloads, at underworldlive.com, among them a recording of “Rez” from a 2005 show in Spain. You need to register to gain entry, and the site’s a bit slow, but it’s worth it. Commercial recordings are housed in the Downloads section, free ones in the Archive section.