My circular saw, a Porter-Cable 324MAG Type 1, has a broken blade bolt. Porter-Cable designed a tool-less bolt removal system that someone broke when I loaned them my saw. This is the bolt that fastens the blade to the arbor. The silver key is supposed to slide to give you a small handle that allows you to unscrew the bolt. Eventually, someone will break it.
Some Googling suggests that a viable replacement is Porter-Cable blade bolt PN 876053 which sells for $9.99 through Amazon with free shipping or $4.67 through servicenet.portercable.com plus 10.99 shipping. I think I'll use Amazon.
These parts are related, but now discontinued:
Porter Cable Locking Blade Clamp (Right-Hand Thread)
Part Number: N37679
Replaces: A02590, A02590SV