How about "they live among us"? Similar to the idea in "frequent hidden contact", except the government isn't aware of it and there is no formal technology exchange, hidden or otherwise. Basically, aliens live on Earth in secret, but earthlings really have no way to regulate their presence or directly benefit from it. Aliens on Earth are probably instead regulated by alien governments (though of course something always comes up to get humans involved in alien business on an individual level).
Also, the classic "aliens show up and demand to meet our leader" scenario is a must have. A total alien invasion would be a subset of that.
Of course, there is always the possibility of a conflict between two alien forces that drags Earth into the mix through accident and coincidence.
I'm not sure if I can think of much else without getting too specific...
