When I need support then I'll go looking for it. For that case, please have a visible link somewhere in the header or footer. There is absolutely no need to stick your 200x400px "SUPPORT!!!"-link into my face permanently. Many of them even perform idiotic gymnastics during scrolling or accidental mouseovers, in order to maximize the distraction.
However, there's one thing even worse than that: Automatic chat popups (Hello Rackspace!).
If you're at the receiving end of such a popup and get some really nasty text from a stranger, then that's probably me.
I'm not a big fan really of any of the support tabs and am sorry they have gained so much traction (thanks UserVoice, I guess?). But thankfully they are not present on the more legit sites.