Encryption on the move

Use Words and Excel

Both of these programs have password systems and this would be the easiest method for you as anyone receiving the files would be able to open them.

 

As ssmed mentions, Office has built-in support for password-protecting files. This is probably the most convenient solution provided you use the latest editions of office (as the functionality is broken prior to 07/08 as far as I know).

zip files also support encryption but I believe it's also broken.

Fact of the matter is, most of these schemes are insecure and shouldn't be relied upon.

Personally, when I need to share sensitive information, I typically do it via the web over SSL (technically TLS). One of the easier ways to do this would be to sign up for S3 from Amazon Web Services and host the files there. You can then access it in a variety of ways (download from their secure web console, create secure presigned urls to give out, or via some third party app). SSL is widely accepted as being secure. Of course, this approach costs a little money, but it also means you don't have to worry about the software on the end-user's computer (besides them having a browser and an internet connection). Of course, this means storing unencrypted data on AWS servers.

I suggest S3 as I judge AWS to be a trustworthy entity. Plus they provide easy access to SSL transport without having to go through the hassle of setting it all up yourself.

 

TrueCrypt is cross platform and free.