The current version, on both the Macintosh and the PC, is now version 7. It should alleviate the numerous transfer problems between platforms we had before. Please visit Adobe site for more information on exchanging Illustrator files between both platforms.
The lingua franca of Illustrator was the format Illustrator for the Mac v. 3. You don't need to save your work as a plain EPS file.
Note that the way Illustrator handles EPS files changed radically from version 5.5 to version 6. Illustrator 5.5 only placed the EPS files, while Illustrator 6 interprets them. That way, you can edit the file. This alone could justify the upgrade cost.
If you are not sure that your partner has the same fonts as you on the target computer, you should convert the text to outlines. This way, the text is not text any more, but just another element of the picture. The command is "Convert to Outlines", in the "Type" menu. Beware that the text is not editable any more after this conversion. Obviously, you would have to make a backup copy of the original file if you want to work on this text.
Illustrator 6 and higher can open most Freehand files (up to version 6). To be more precise, version 6.0.1 owner can do it direct out of the box, while version 6 needs a plug in, available from the download section of Adobe Web site.
However, the file format for version 7 of Freehand is not the same any more, so you have to find another solution.
Note also than Freehand 5 (now a product from Macromedia) can open directly the files created in Illustrator 5 on the Macintosh.
There are tools to convert DXF files (standard format of most CAD programs) to Illustrator. The Web site (AutoSET) of John Walker Graphics Pty Ltd. presents information on these tools and we recommend you visit it.