I assume that when you say "access" you mean both r/w and you probably want to be able to set permissions and do everything else you can do on your regular HDD.
Linux supports more file systems than OSX, but there is a smaller overlap than you would think.
The file systems OSX "officially" supports are:
FAT 16 and 32
Linux is not really compatible with UFS. While it is compatible with HFS+ you have to sacrifice stability for it. FAT32 is not case sensitive which is a day-to-day user-level problem for OSX and linux users who are used to it being case sensitive. It also lacks a number of features that modern file systems have like symbolic links and journaling.
I'd recommend ext3. That is the default file system for quite a few linux distros for a while and is compatible with OSX as well.