Not sure what you mean by "accessible", so I'll go from easiest to least easy:
Truecrypt has been audited, works on Windows, and appears to offer an encrypted file container that you can attach to a Windows drive letter.
What you could do is create the container, let DropBox (or whatever) sync the file containing the container, then perform your file operations within the container.
Depending on your needs, there's also GPG4Win, which comes recommended by the GPG project maintainers. AFAIK, GPG hasn't been subject to an extensive audit, but it is venerable, open source, and developed in the open. (Yeah, so was OpenSSL, I know. ;) )
I don't know that there's a way to make a transparently encrypted container with GPG as there is in TrueCrypt, so I suspect that you'd have to:
* Copy your encrypted files
* Decrypt them
* Edit them
* Reencrypt them
* Copy them to the sync directory.
Sounds like a bit of work to me.
If you were on Linux, you could probably use dm-crypt on a loop device, and just sync the underlying encrypted file, but -again-, I have no idea how much auditing has been done on the underlying code (and this fails your "for Windows users" requirement).
Oh, BTW, I replied to our Erlang thread. If you're interested in taking a second look at Erlang, I link to a few things in the comment that might be interesting to you.