AxCrypt is open source software that can create self-decrypting files.
It doesn't know anything about handling .zip files, but having a Zip utility is pretty much universal.
However, I can see that if you're sending the files to very non-computer savvy prople (like everyone in my family), the Chilkat tool would be nice. I just get a good feeling from having the source to AxCrypt available (even though I've never actually done anything with it).