Right now I store it on DropBox, or my inbox, but that doesn't seem very secure.
In the past, some people have commented about placing a TrueCrypt volume-as-file on DropBox. Apparently performance and storage consumption were, per them, not as bad as one might think. (Think about the potential implications of the version history that DropBox provides by default.)
There are some other programs, including some text editors, notepad type editors, and "personal wiki" programs that offer to save to an AES or Blowfish encrypted format. I imagine the correctness/robustness of their implementations may vary.
Finally, if you just want to encrypt a file already disk, the well regarded archive program 7zip (and others) includes an AES encryption/decryption implementation. I don't know how its implementation is regarded. Note that, under DropBox, this won't eliminate prior, unencrypted versions that are saved by its versioning system.
Also, GPG and PGP are always around.
If you encrypt a pre-existing file, outside of the DropBox context it's a good idea to encrypt to a fresh copy, then securely wipe the original.
EDIT: Depending upon what is being used for encryption, the DropBox versioning might itself represent a weakness. I'm not qualified to speak to such topics -- just imagining.
At home I really like 1Password for the Mac except that the version I have doesn't work with Safari 5 or Chrome.
For secure files on my Mac, I keep a password-protcted & encrypted sparsebundle disk image in Dropbox. You can create one using Disk Utility. It's really handy and always backed up.
I can't speak to proving the security of these solutions but work for me otherwise.
A Windows version just entered beta.