There is no good file format that works with Win / Mac native, except Flash. Even Flash actually needs to go online to download the software.
The latest version of Windows media supports some QuickTime formats, but it'll be a while until everyone has it.
The best way to go about this for future projects would be hire a programmer who could set you up with a simple online video preview area.
No more worries about size in email attachments and the client can easily forward the link to anyone. Another is to setup a private Youtube account (limited to 10 minutes).
Or if you have 800 bucks to blow, take a look at
MediaBatch
Other than that, either ask the client what they prefer: Mac or Windows format, then render a QuickTime H.264 or a Windows Media file, or both.
Vince Becquiot
Kaptis Studios
San Francisco - Bay Area
