Windows only: Encrypt and compress documents, data and Outlook e-mail with SecureZIP, a free program from the makers of PKZIP.
The software supports encryption via passphrase, digital certificates or both. Normally a digital certificate will cost you, but the SecureZIP tips guide (PDF) lists sites where you can obtain free ones. Encrypting files on your PC is as simple as creating a new ZIP file, which you can do by dragging and dropping, right-clicking, etc. The program also features Outlook integration, thus allowing you to protect both the body and attachments of outbound e-mail. (These messages will include an instruction file so the recipient can install the necessary ZIP reader.)
Needless to say, SecureZIP also serves as a ZIP manager; you can compress and extract files without encryption, making this a pretty attractive alternative to the ugly-but-popular open-source zipper 7-zip.
What bugs me about SecureZIP, however, is that the site where you obtain your download code offers zero information about the software's features, system requirements, etc. The aforementioned PDF fills in some of the blanks, but if you want the full scoop, you'll need to visit PKWARE's main site.
SecureZIP requires Windows 2000 or later. It's free, not trialware, though the site does mention it being a limited time offer. So get it now if you want a pro-level ZIP manager and encryption tool.