This blog post by Eugene Spafford points out that

Microsoft Word:
  1. Is not a document interchange format -- it is not designed for document transport
  2. Is not installed on everyone's machine, nor available for everyone's machine
  3. Not all versions of Word are compatible with each other
  4. Results in huge, bloated, files for tiny content (such as memos)
  5. And of course, Word is commonly a vector of viruses and maicious hacks.

He includes a link to the (plaintext) "bounce message" that he uses to educate people who send him Word documents. Well worth a look.

Similar considerations apply to files produced by PowerPoint, Finale, Photoshop, and other programs. Open-source, cross-platform programs like Audacity and the Gimp aren't immune either: even when they're available cross-platform, you shouldn't use them for email.

If you're actively collaborating with somebody who you know is using the same program and version (I'm upgrading to Audacity 1.3; this isn't just a Microsoft problem by any means), by all means use that program's file format, but put it on a website and email the URL rather than trying to ship the whole darned thing in email. Many email systems will bounce big files anyway.

If you're sending a finished product, use plain text, HTML, or XML if at all possible. Other text-based standards include LaTeX for typeset documents and ABC for music. Images can be sent as PDF or JPEG, formatted documents as PDF, music notation as a zipped MIDI file, and sound files as FLAC or Ogg. But even here they're big enough that you'll want to put them on the web and email a link.

(via .) ETA Oh, and if you're thinking this was inspired by a particular piece of email, or a particular blog post, it wasn't. Just seemed like a good reminder, especially now that I'm trying to put together a CDROM.