I'd say bands with "a certain old school sound" too.
Basically bands from the 70's and 80's that share a common sound or time frame, as well as bands from the 90's and today that share a common sound:
70s: Black Sabbath, Deep Purple, Rainbow, Thin Lizzy, Judas Priest, Motorhead, Rush, Uriah Heep, Riot
80s: most of the suspects above, almost all NWOBHM, classic thrash bands, classic doom bands
90's: power metal bands (both Euro and American species)
today: Wolf, Enforcer, White Wizzard, Grand Magus, Black Tide (debut), Final Stage, Stimulans, many newer thrash bands and the retro movement (Ghost, Spirits Of The Dead).
I guess that old school also depends on your age. If you're 50 old school are the 70's, if you're 15 old school are the 90's and the 70's is the mesozoic era
P.S. Oh silly me, I forgot Sabaton