Comic Sans -- the font everyone (well, almost everyone) hates and everyone has. Being installed almost universally and being a quite legible font for online applications, it does get used. But let's face it, Comic is somewhat ugly and may imply a certain tastelessness in its users.
Especially when combined with garish colors!
The one place I might use Comic? As really small text online. It's surprisingly readable as small as 8 point and looks better when reduced in size, as well -- less cartoony and more script-like. Still, I'd probably choose Verdana for pretty much any type that small.
Not that I ever go smaller than 10 pt on my own sites. There is no reason to strain folks' eyes and it doesn't cost anything to have a larger page!
If I needed a Comic-like typeface I would probably opt for Dom Casual. It may not as suitable for on-screen readability but is decidedly better looking as a display font.