They appeared a bit with Superman in the early '00s (although I think the Linear Men originated pre-Crisis). The Linear Men were a group of time travelers who went through time trying to fix paradoxes and anomalies and such. They met Waverider, a guy who was turned into living tachyons (a.k.a. magic macguffin time travel particles) in the Armageddon: 2001 event, and it turned out that the leader of the Linear Men was another timeline's Matthew Ryder, the same guy who became Waverider. They initially clashed, but they realized their goals were the same--to protect the timestream--so they teamed up and made Vanishing Point their HQ. Rip Hunter was once a member of the Linear Men, although I think as of Booster Gold, he took it upon himself to shut the Linear Men down for some reason.