My bike's been left in the rain so much when locked up at work that the smaller steel nuts and bolts are all orange with rust. But it's the chain that's really suffered. Not so much rusty, but it's become a bone dry squealing thing as the rain washed away all the oil. So I bought some fancy chain oil.
And not just any chain oil; specific oil for wet rainy conditions. As I bought it, I told the fellow who runs the Europe Bound, "If I buy this, it won't rain any more!" Sadly, this proved incorrect. Some 10mm came down today. Happily, I was able to find a space indoors to park my faithful steed.
(I may be imagining it, but the fancy chain oil turned out to be much more effective than the stuff I usually use. Even used sparingly it's made a huge difference.