TinyPKC is a port of the CyaSSL implentation of RSA and ECC to TinyOS 2.x that is released under the GPLv2. It was tested on 16-bit and 32-bit microcontroller platforms and should run on 8-bit platforms as well. It supports ECC key lengths from 112 bits to 521 bits and arbitrary RSA key lengths.
Some performance figures for an Atmel SAM3U microcontroller:
And some ECC figures:
Source code is available here.