Why isn’t carbon monoxide a free radical? By the rules of valence (which I know don’t perfectly describe how atoms combine), one carbon atom with one oxygen atom should leave two unlinked convalant bonds. Yet CO is stable, doesn’t polymerize with itself or spontaneously combine with O2 at room temperature. Why CO and not, say, CH2?
A quick search seems to indicate the molecular orbitals are shared or overlap somehow –
Try this:
http://www.madsci.org/posts/archives/oct98/906435573.Ch.r.html
I was able to find several other pages but I had to fix the search string: +“carbon monoxide” -detector -sensor -poisoning +bond
As you might have guessed, a lot of carbon monoxide references on the web have to do with its role as a pollutant and asphixiant.
“Finally, consider Kottke’s voice which sounds like geese farts on a muggy day.”
Leo Kottke
6- And 12-String Guitar
CO isn’t a free radical because it doesn’t have to be. CO forms a triple bond between the Carbon and the Oxygen with a lone pair of electrons at each atom. Using the Localized Electon Model each lone pair is sp hybridized. The triple bond consists of a sigma bond produced by the overlap of an sp orbital from each atom and two pi bonds produced by the overlap of 2p orbitals from each atom. You can also use the Molecular Orbital model to see that it is stable with a bond order of 3.
This thread reminds me why my excellent and easy A in HS Chemistry became a rather desultory B in College, and why I vowed NEVER to investigate such courses as PChem or Organic… 
If you’ve noticed that Squid’s answer isn’t the same as the page that pluto posted the link to, there’s a simple explanation:
It’s because Squid’s answer is correct.
The other answer is… well, not exactly wrong, maybe, but bizarre.
Sorry if I misled anybody. I don’t know enough about chemistry to answer the question myself. I just found the webpage and assumed it was correct. Won’t happen again! 
“If you had manifested fatigue upon noticing that you had been an ass, that would have been logical, that would have been rational; whereas it seems to me that to manifest surprise was to be again an ass.”
Mark Twain
Personal Recollections of Joan of Arc
Not your fault, pluto… there ARE some things on the page that are more or less right, and the author does claim to have a Masters degree; if you didn’t already know the answer, it would be pretty easy to believe what it said.
Triple covalent bonds with oxygen atoms? “SP” orbitals? Bonds named after Greek letters?
Boy oh boy, has chemistry changed since I was a lad.
The truth, as always, is more complicated than that.