One thing that should be clarified is that it is a fact that Saddam Hussein's Iraq had WMD's at one point. We know this, because he used them on his own Kurdish population in 1988. At issue in 2003 was if he still had them, or if he'd dismantled them all, and his production program, as he'd agreed to do at the end of the first Gulf War.
Now, it would perhaps be easier to list who didn't think Iraq still had WMD's in 2003.
One name on that (much shorter) list would be Hans Blix, the guy in charge of weapons inspections for Iraq. In other words, the person most in a position to make that assessment. He made a statement before the war started that he thought they pretty much had all of Iraq's WMD source material accounted for. This was much derided by the right wing in America at the time.
A large part of the problem was that Iraq acted like they had WMD's. They were constantly refusing inspectors access to certian sites, then waiting a while and letting them back in. They were refusing access to scientists. They were just not cooperating in a way that didn't make much sense unless they had something to hide.
The explanation I've heard for this since was that Hussein felt like the weapons were a deterrent to Iran, and wanted Iran to believe he still had them. Sadly for him (and many others), he ended up decieving the wrong people.