I love this class of utility -- keeping data mobile.
In Mac OS X 10.4 'Tiger', the default message format for Mail messages changed from the Apple custom mbox-package format to the new emlx format (where messages are stored in individual files for Spotlight indexing). However, if you need to recover from a hard drive crash, it's almost impossible to recover your mail messages easily since Mail won't import emlx files and you can't add them to your mailboxes any other way.
This tool will convert your individual emlx mail files (found in ~/Library/Mail/) to the old mbox format, used by almost every UNIX/Linux mail client and recognized by many more.
With this tool, you could convert some individual emlx files to mbox and then import the mbox file using Mail for Mac OS X or almost any other mail client.
To use the tool, drag your emlx files into the main window. Click the 'Save mbox...' button and a prompt will appear, allowing you to save an mbox file.