If you are exchanged email messages with people using Linux, Windows and some odd Mac and want to implement some really basic security, encrypting all messages, just trying to avoid some non-sophisticated attacker. Is there a tool already installed on these systems by default? Alternatively, a compatible procedure that won't require users to install additional software or understand fancy concepts will also do the trick. It doesn't have to be the same tool exactly, just compatible. Like being able to open a .doc file with MS Word and Open Office.
In short: no, there's nothing that's guaranteed to be installed on those operating systems.
PGP is probably your best bet. It operates on the OpenPGP standard, which any software is free to implement. There are compatible clients for Windows, Linux, BSD, and OS X, and it also nicely integrates with most mail applications (e.g. Thunderbird / Outlook). Many of these applications and plugins are free or open-source, though some are commercial. However, it's not quite as simple as just setting a password; PGP operates on the idea of each user having a key that identifies them, and encryption involves using the target user's public key to secure the data. As such you need to be able to pre-share the public keys (e.g. via a key server, or just by sending it as base64-encoded text) in order to perform encryption.
One alternative is an archive format that supports inbuilt encryption, but finding compatible software across all operating systems is going to be a pain. 7zip might be a good choice, but I can't say I've got any experience with it on OS X. Other formats like PKZIP (.zip) and RAR tend to have discrepancies in the way that encryption is done, not only across operating systems but even between tools.