This blog post by Eugene Spafford points out that
- Is not a document interchange format -- it is not designed for document transport
- Is not installed on everyone's machine, nor available for everyone's machine
- Not all versions of Word are compatible with each other
- Results in huge, bloated, files for tiny content (such as memos)
- 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.