Every once in a while, someone will ask me what I use for keeping passwords securely. I tell them that I use password safe, which was reccommended to me when *I* asked the question.
Other times, people will ask for simple ways to encrypt or store files. If you're looking for something robust, cross platform, and full featured, you could do a lot worse than TrueCrypt. Essentially, it hooks into the operating system's kernel and allows it to mount entire encrypted volumes as if they were drives. It also has advanced security methods to hide volumes, so that if searched, no volumes would be found without knowing the proper key. In addition, it has a feature that can be valuable if you are seized and placed under duress: in addition to the "real" password, a 2nd can be setup to open another volume, so that your captors believe that you gave them the correct information. Unreal.
So you see that truecrypt is an amazing piece of software. For many things, it's definitely overkill. Instead, you just want something light, that will encrypt a file and that's it. In this case, Gnu Privacy Guard is probably your best bet. I use it in our company to send and receive client files over non secure transfer methods (FTP and the like). With proper Key Exchange, we can be absolutely sure that a file on our servers came from our clients, and vice versa. If you're running a Linux distribution, chances are good you've got GPG installed already. Windows and Mac users will have to get it, but it's absolutely worth it, and the knowledge of how public key encryption works is at the heart of everything from web certificates to ssh authentication. If you want to learn more about how to use it, Simple Help has a tutorial on it, covering the very basic usage. Once you're comfortable with that, check out the manual.
I'm sure I missed some fun ones, so make sure to suggest what you use!