Sorry for my late reply. I know that this was mentioned but here goes:
FTP: - setup an FTP server on the Linux box. Make sure its configured properly with permissions et all. As mentioned, through finder it will be read-only. You can install a 3rd party app, or you can drop to the command line and use the ftp client there.
SAMBA: - most Linux distros have Samba installed. Its not overly complicated to setup if it is not installed.
http://www.samba.org is where you want to get more information from. Using Samba, you can setup windows shares, and then connect to them through Mac OS X. And yes, you can make them read/write
NFS: Although this isn't a popular option in most linux distros - you can get your hands dirty and setup an NFS server. Basically setup your exports on the linux side, then mount them on the Mac OS X side. NFS is quite efficient if configured properly.
X: One of the great things about Mac OS X is that you can run X
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You can configure XDM on the Linux machine to accept remote connections. Then you can connect to the Linux machine from your Mac OS, and get the Linux desktop .. gnome - whatever you want. This is more than getting your hands dirty of course.
Other ways.
* You can setup a kerberos realm on your Linux machine, and then do away with authentication when connecting from your mac box to your linux box .. Its essentially a single-sign on system.
* You can set up a directory server using OpenLDAP, and then get your Mac OS X box to authenticate off of your Linux system. You can even mix in kerberos, NFS, the works - When you login to your Mac, it'll authenticate off of your linux box's OpenLDAP, mount your home directory and other mount points via NFS, and populate your Dock with X11 Apps that execute remotely from your Linux box. Oooh the fun
Short answer is that you can do a lot of networking between Mac OS X and Unix like operating systems - they will integrate together very nicely --- depending on how much you want to delve into it
