http://www.digital-scurf.org/software/libgfshare
https://github.com/ryancdotorg/threshcrypt (uses libgfshare, written by me - handles large files by splitting a key into shares and encrypting them with password-derived keys)
https://github.com/amper5and/secrets.js/
http://passguardian.com/ (uses secrets.js)
http://point-at-infinity.org/ssss/