Forgotten heavy bands from late 60's to early 70's

Some more familiar names from the early 70's:
Uriah Heep
Try Live, Demons and Wizards, the Magician's Birthday
Wishbone Ash
Try Argus and live Dates.
While not as acid drenched as 13th floor elevators, these two are definately very heavy. Uriah Heep Live is an all time classic and Wishbone Ash was one of the first bands to use a dual lead guitar approach which definately inspired Iron Maiden, Priest, etc.

Hawkwind and Spirit

Not a Heavy Metal band but a Heavy Classic Rock band, Spooky Tooth made a classic album, "Spooky Two", great album. That's the only one I can recommend. They put out others you might want to explore if you like Spooky Two.

I always thought Witchfinder General was pretty cool. They sounded a lot like Sabbath

-Stongly second the 1st Sir Lord Baltimore.
-1st Trettioariga Kriget (genius brain melting stuff)
-John McLaughlin - Devotion
-Patto 1st
-High Tide - Sea Shanties
-Budgie - 1st or Squawk
-Steamhammer - Speech, (also check out Armageddon)
-Captain Beyond
-Skid Row (w/ Gary Moore)
-Guru Guru - UFO
-Groundhogs - Split
-Pink Fairies - Never Neverland
-Deviants 3
-Ursa Major - Ursa Major (Dick Wagner before R&R Animal)
Fortunately there are lots of others... have fun hunting em' down!