This tool does public key encryption and decryption. It does the entire process in the browser. No data, keys or crypto are sent to any server. (Check the developer panel network tab for confirmation.)
1) Generate a key pair.
2) Extract a public key from a private key.
3) Manually enter public or private key.
4) Create a crypto text string from plain text input.
(Requires entering either a public or private key or both.)
5) Extract plain text from a crypto string.
(Requires entering a private key.)
This tool is based on the excellent node-rsa and Browserify. Read more and get the code from this blog post and github. Written by TQ White II.
Quick Start: Click 'Generate Keys', Click 'Encrypt', Click 'Decrypt'. (Also, watch this 45 second video.)

Private Key
Public Key

Plain Text Input
Generated Crypto Text

Crypto Text Input
Decrypted Plain Text