Back in beta, we used the Nether as a highway system. Every one block you travel on the x and z there is equal to 8 blocks in the overworld. That was loads of fun to build. I've played on worlds that really, really like railroads, but thats never been my thing. I don't know why, but its just not interesting to me.
On a server I played on before the one I run with family, we used ic2 teleporters. The world was limited in size to a 2000 block radius, which kept costs from getting too insane, but it was still expensive to travel between the central hub and whatever base you built outside. That made for some fun projects.
In our current world, we've been making extensive use of linking books for long range transportation. We try to minimize the portal gun usage due to lag. I've been contemplating ic2 teleporters for in-base movements, especially to maintenance areas (using a gregtech player detector to only allow myself into them) or accessing the more complicated circuitry...
