MacOS – ny Mac software to handle archives (zip, rar, 7z, tar, etc.) that allows browsing

Not aware of anything quite like BeTwin, but you may not be aware that is is possible to run 2 user accounts simultaeniously out of the box without the need for any extra software, at least in Lion (And I have seen, but cannot find, that this can be retroconfigured on SL). You mention SLin your question, but have also tagged, Lion, so I will reply anyway.

When you use screen sharing, you are able to choose which display to connect to, either the physical or the virtual display. Connecting to the physical display will allow you to take over the screen of the target computer, taking control of the kb/mouse etc. But if you use the virtual display, you are able to log in as a new user, and run processes entirely independently of the peron who is physically logged onto the machine.

Whilst I have only tested this with a a Mac initiating the screen sharing, there is no reason why with an up to date VNC client any old netbook, or thin client etc shouldn't be able to connect in this way, as the Screen Sharing technology is only VNC under the hood.

Edit: Read this for more information on getting this working using Snow Leopard

OS X can handle both by default from Finder. The Archive Utility that ships with OS X knows how to extract both formats. Just double click on the file or Cmd-Click (or right click) on the archive and select 'Open' from the list and OS X will unarchive it.

Users also have access to zip, unzip and tar from a Terminal window command line. These all come with OS X.

There's also the free, and most excellent, Unarchiver that they can install that allows for a few more options when dealing with compressed files. It also supports a few more formats. But it's completely unnecessary for extracting gzip-compressed tar archives and zip archives.