Procol Harum: Shine on Brightly - hell, the band as a whole is underrated, IMO.
King Crimson: In the Wake of Poseidon - Often discounted as a rehash of "In the Court of the Crimson King", but there's a lot of good music to be heard.
AC/DC: Let There Be Rock - completely overshadowed by other Bon Scott era classics like Dirty Deeds and Highway to Hell.
Santana: Caravanserai - First Santana album to incorporate Jazz and with spectacular results. Underrated by the masses because it "wasn't Rock".
High Tide: Sea Shanties - Never heard of them? I hadn't either. But this is some gooooooood hard rock with a little bit of progressiveness to round out the package. Lineup is guitar/vocals, Drums, Bass... and electric fiddle!
Emerson, Lake and Palmer: Tarkus - Just doesn't seem to get the attention or love it deserves, but it's a fantastic album and worth the price of admission for the first track alone.
The Kinks: Kinda Kinks (and any "Marble Arch" recordings) - People seem to consider the Kinks a simple pop/garage band, but the influence of this band on contemporaries is undeniable and their lyrical content was excellent. Beyond that, Dave Davies just knows how to kick out some great 2 minute rock tunes.