"Filler," songs are generally those songs that are thrown together at the last minute to fill out an album. More often than not they are mediocre at best (although sometimes they are quite good).
Every now and then, though, a filler song ends up being one of the best tracks on the album it appears on.
Here are a few examples:
Deep Purple - "Smoke on the Water" - This song was taken from a studio jam that the band occasionally played as a warm up before doing recording. The song was fleshed out and then given lyrics which reflected exactly what had happened over the previous few days (the casino/theater fire, search for new recording venue, etc). But despite the current status as Deep Purple's biggest song they thought that it was a throwaway, and that, "Never Before," would be the big hit.
Thin Lizzy - "The Boys are Back in Town" - Like "Smoke," this was a song thrown together to fill out an album. Thin Lizzy bandleader Phil Lynott thought so little of the song that at first he didn't want it included on the Jailbreak album. This would become a massive hit and their signature song.
Black Sabbath - "Paranoid" - Another song thrown together, only this one literally at the last minute. They threw this one together in less than an hour.
What are some other, "Filler," songs that became classics? I'm sure I'm missing quite a few.