Led Zeppelin's 'Houses of the Holy,' which is No. 42 on our countdown of the Top 50 Led Zeppelin Songs, would rank as the greatest B-side in rock history -- that is, if it hadn't shown up on their 1975 sprawling double-album, 'Physical Graffiti.'

Lean, hook-laden, and lightly funky, the track was originally recorded for the band's 1973 album 'Houses of the Holy' (makes sense, right?), though it was left off at the last minute when the band decided it wasn't a proper fit in the context of that album. When Led Zep realized that, for their follow-up, they had enough material for two releases, they considered some of the material they'd discarded from earlier sessions. Luckily, 'Houses of the Holy' made the cut. It's one of the band's catchiest and deceptively weird tunes.

Not many Led Zeppelin tunes are this sparse: John Paul Jones and John Bonham offer a linear, minimal groove, while Jimmy Page lays down a blues riff that ventures into some strange territory with the use of jazzy chord shapes and noisy effects pedals. There's plenty of space for Robert Plant's soulful hook--one of the most instantly hummable in the band's discography, even as the lyrics bounce from playful to Satanic. 'There's an angel on my shoulder / In my hand a sword of gold,' Plant sings over the mid-tempo crunch, 'Let me wander in your garden / And the seeds of love I'll sow.'