SJG's Killer is currently available in a new print and has been through several editions in several languages. The 1st edition from 1982 (below) used to be much harder to come by, but I've seen several copies floating around ebay lately. The 2nd edition from 1985 is a bit more common. (I think they may have re-published to ride the coattails of the movie Gotcha that same year.)
The 1st ed. of Mechanoid Invasion (below, Palladium's first real RPG) is also a little hard to come by, but the collected Mechanoid Invasion Trilogy is fairly readily available.
As for Omnigon, it's not terribly obscure, but not widely known. Honestly, if it weren't for games like Spawn of Fashan and World of Synnibar being so widely known for being so bad, Omnigon might be better known.
On a side, but related, note if any of you live in Austin (or nearby): A guy from one of the Half-price Books there (S. Lamar location) said that about once a month a woman from Steve Jackson Games comes in an dumps a bunch of old RPG stuff from their archives. (Don't forget to check the vintage/memorabilia section, not just the game shelves. And let me know if you pick up anything cool. I'd already spent my limit the last time I was there, or I would have bought The Compleat Adventurer and The Compleat Alchemist.)