I am a long time user of Orthodox File Managers, which can generally be categorised as a two pane view of a hierarchical storage system. Examples of these include file, registry, archive (zip, rar, etc) ftp, nfs, etc. I used Far for some time, which has a fantastic plug-in model, but was not progressing. I changed to Salamander 2.5 Beta well over a year ago, and have never looked back. Now that it has been released, you can pick it up for $30 USD.
Whilst Far is somewhat nostalgic of DOS file managers, like Norton and XTree, we live in a world of WYSIWYG, and I find character mode doesn't cut it when previewing files. This is where the move to Salamander was welcomed. It's still missing features a loved in Far, but if Altap ever release their SDK, I'll be the first to jump in.
For OSX I have been experimenting with Disk Order 2.5 - it still has a ways to go, lacking many of the standard key strokes one expects from a 2 pane file manager, but is by far the best option on the Apple platform today.