Far too many to mention. The Wikipedia entry (link above) is quite comprehensive - lots of movies are just cheaply and badly-made, scripted, acted, directed etc. but that's not really what makes films bad.
What really makes films bad is quality resources and still downright awful movies, so big Hollywood films which are just awful are the real worst of the worst.
I've personally avoided seeing many movies which critics have panned because I have better things to do with my time, even if it is only 90 minutes or so. So I'll never, ever watch a movie with Rob Schneider or Tim Allen in, for example, because you just know they're going to suck.
Probably the worst film I've seen all the way through (in a vain hope that it would actually improve) was Freddy Got Fingered. It was just unbearably bad.