Don't add links, just talk about some Microbe games you've found online or so.
Dr Mario is a pretty famous one. Not playing as a microbe, but a Gm-like bad guy.
Spore has a cell stage, where you can create your own cell, with eyes and such, very Gm-like. A more in-depth version called Spore Origins is available for iPod Touch, iPhones, and Cellphones.
Plenty of online games I found. One called "Pandemic," where you play as a virus/bacteria/parasite and take over the world. One I forget the name of you play as a virus while infecting Red-Blood Cells, while avoiding Antibiotics and White-Blood Cells. Another one you play as a nanobot and do puzzles to stop an infection.
I think I like Spore best, because it's the most creative. The microbes in that game or Spore Origins remind me of GIANTmicrobes. Here's an example:
Can you guys think of any more?