Cryptonite brings EncFS and TrueCrypt to Android. You can browse, export and open EncFS-encrypted directories and files on your Dropbox and on your phone. On rooted phones that support FUSE (e.g. CyanogenMod) you can also mount EncFS and TrueCrypt volumes. TrueCrypt is only available as a command-line version at this time.

Cryptonite is available from Google Play. Bleeding-edge alpha versions are sometimes available here. Report bugs to the issue tracker.

What's new? See ChangeLog.

Read up on important security considerations here: SecurityConsiderations

Why can't I decrypt and encrypt on the fly? See FrequentlyAskedQuestions.

Want to build your own apk? See BuildInstructions.

Encrypt your Dropbox with EncFS

GNU/Linux

This is simple and straightforward. The default packages should work just fine. There's a nice tutorial on Webupd8.

OS X

Fuse4x + EncFS worked well for me on OS X 10.7, as described in these tutorials:

Google-translated tutorial on lisanet.de

German original on lisanet.de

Another tutorial on webdiary.com

MacFUSE + EncFS might work as well, but I haven't tested it:

Google-translated tutorial on joern-ende.de

German original on joern-ende.de

Yet another tutorial on packetslave.com

Windows

encfs4win works fine for me.