I use this (http://www.imakproducts.com/product.php?c=Computer+Comfort&s=16) mouse rest daily at work.
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(http://www.imakproducts.com/img/products/clips/16_2.jpg)
I totally recommend it. It is very comfortable. It uses very small beads instead of gel or foam.
On the other hand, i don't recommend this (http://www.imakproducts.com/product.php?c=Computer+Comfort&s=15) wrist rest from the same guys. My wrists sink in it and it interrupts my typing.
The mouse wrist rest is a good idea since you move the mouse around. I would get one but I use multiple computers and have no "main" machine.