A couple companies are making mini mice for laptops with small transmitters. [link|http://www.targus.com/product_details.asp?sku=PAUM005U|Targus mini mouse] I went for the Micro Innovations mouse; the transmitter is a bit bigger, but it included a rechargeable NiMH battery. So far it's been good.
For desktop systems, I use Logitech.
Tony