Types of Cryptography for a 4-8 bit microcontroller This is more of a research question.

I was wondering what types of crypto algorithms would work best on a small 4-8 bit micro controller.  I recently read a paper called Security Considerations for 802.15.4 Networks and was wondering if anyone out there can think of any other papers or has knowledge about what types of things to consider when implementing cryptography on a low-power, low-memory system. 

I read a 2009 ieee paper on implementing RFID on a 4-bit micro controller, but I am looking for papers that will generally apply towards 802.11 or 802.15 standards.  I will propablly be trying to hook a bunch of these controllers together, so I am looking for more of asymmetric encryption.
Public-key encryption on microcontrollers

Erik-oliver Blaß ,  Martina Zitterbart.
"Towards Acceptable Public-Key Encryption in Sensor Networks". 2005.

"dsPIC DSC Asymmetric Key Embedded Encryption Library"
an implementation of RSA, Diffie-Hellman, DSA, SHA-1, MD5.
(Are such 16-bit microcontrollers in your range of interest?)

"Links to Embedded Crypto Implementations"
lists a few implementations of RSA on 8 bit microcontrollers.

"full size" symmetric ciphers on microcontrollers

Many people implement cryptography on a microcontroller very similarly to cryptography on any other CPU -- write a program to do encryption or decryption using the same ciphers we use on any other CPU.
Quite a few modern 8-bit microcontrollers have enough program memory (often called "ROM" for historical reasons, although today it's usually Flash memory) and RAM and processing speed to handle the full Advanced Encryption Standard (AES)(a)(b)(c)(d), Triple Data Encryption Standard (3DES)(a)(b), Enigma(a), RC4(a),
as well as less-known algorithms 
such as Alexander Pukall’s PC1 (a).

symmetric ciphers designed for low RAM

Quite a few microcontrollers have 256 bytes of RAM or less. A few full-size ciphers work just fine, but ciphers like RC4 seem impossible to implement in this environment.
A few symmetric ciphers seem specifically designed for this environment -- KeeLoq, TEA, XXTEA, etc.

Other cryptographic techniques for microcontrollers

A few bootloaders for embedded systems use encryption.( "USB PIC Bootloader with XTEA encryption" )

Rather than doing the encryption in the CPU itself, a few systems have an external chip that does crypto stuff.
( CryptoAuthentication, CryptoMemory, Trusted Platform Module, etc. )
Such dedicated circuitry, hard-wired to perform some calculation, often requires significantly less power than a more general-purpose CPU programmed to perform the same calculation.
This way newly-introduced bugs in the software on the main CPU can't possibly leak the secret keys stored in that external chip, or introduce new flaws (new timing attacks, etc.) in the algorithm implemented in that external chip.