Iron Crown used to publish some wonderful campaign settings/supplements, both for use with their Rolemaster line and for use as a generic supplement (not mention Fantasy HERO and MERP, mind). This Camapign Classics line remains a very well-regarded part of Rolemaster's legacy. I recommend that if you find copies of some of the titles below, you snap them up, as they stand next to many of the GURPS sourcebooks as excellent supplements:
Robin Hood: A nice fleshing out of Norman England, from after the Battle of Hastings to the rule of King John. As you may expect from the title, the legendary Robin HoThere's some good advice and rules for running an outlaw campaign, and some adventures to round the product out. Really, the rules are the worst part of this product--the source material shines, especially if you love history like I do. At this time, you'll need to find an old print copy, as Iron Crown does not currently offer this title as a pdf.
Vikings: This books was made in two versions: one for Rolemaster, and one for HERO. store. I love the thought of Viking campaigns, but this book wasn't as good as other Viking supplements, such as the one from the RuneQuest line. This is probably the "weak sister" of the three books presented here. Again, good historical content, but I don't know that we ever used the rules portions. Like Robin Hood, this book does not appear to currently be available as a pdf from Iron Crown's site.
Mythic Greece: As with Vikings, two versions of this book (one HERO, one Rolemaster) were printed. This is not really a historical guide, but instead details a Greece passed down to us from myth and legend. I remember using the Hero and Demigod rules to great effect, for playing divinely favored, iconic heroes. The monsters, the cultures presented, and the emphasis on deific interference really help flavor this product for a heroic, fictional Greek campaign of legend. This is one of my favorite books of the lines, and really helped me in planning a campaign involving deities and epic quests that didn't just feel either impossible or a total cakewalk.
Happily, Mythic Greece is available from Iron Crown's online store.
Other supplements in this vein that I did not personally get a chance to check into include Mythic Egypt, Arabian Nights, and Pirates. Pirates seems to get a lot of love for its ship design and combat, though I cannot attest to it personally.