True examples of slippery slopes:
income tax - see write-up. hell, taxes in general
gun control - Registration often leads to confiscation, no matter what politicians may promise (of course, depending on your views, this could be a good thing)
social security number - it was originally intended and promised as only for social security - try living in the US without giving yours out nowadays
self-regulation - corporations love the idea but (as much as i hate to say it) it doesn't seem to work (see right to privacy)
ratings - they start out advisory only but politicians seem to like the idea of making them mandatory
censorship - radical feminists backed censorship laws in Canada to get rid of evil porn - guess whose books started getting grabbed by customs officers?
There's got to be others, and this is pretty North America-centered - any others?