RC4 Encoder is a simple and fast stream encoder with quite weak security.
The complete algorhithm description can be found here.
The usage is quite simple :
String EncodeWithRC4(String key, String data)
There are also encoding functions for in-place buffer encoding and for out-of-place buffer encoding; you can also give the key as a buffer/length couple.
See the source code (RC4.h) for details.