I use GPG (Gnu Privacy Guard which is freeware PGP). You don't need to generate a keypair either.
To encrypt a file called...uh... nature-photo.jpg do this at the command prompt:
gpg -c nature-photo.jpg
It will then ask for you to enter a password twice and will create "nature-photo.jpg.gpg"
To decrypt it's just:
gpg nature-photo.jpg.gpg
You will have to delete the original file after you create the encrypted version and this obviously does not hide file names. You can rename the gpg file if you want (to help hide the content type for example), but when it's decrypted, the decrypted file name will be whatever is the filename of the .gpg file, not the original file name. Example: renaming nature-photo.jpg.gpg to stuff.txt.gpg...the decrypted file would be stuff.txt.
Something I'm waiting for is the Mac version of TrueCrypt (
www.truecrypt.org). They claim to be working on a Mac version, but that claim has been out there for a long time with nothing to show for it...not even a beta. The Windows version is excellent and the Linux version is ok (only ok since it only has a command-line interface, but I think there's a 3rd party GUI add-on for it).
Now, for Mac-only stuff, I just use an encrypted .dmg since it's easy to do