for Collections usable for dropdowns, @Factory and @Unwrap is the one I can think of.
For a selection in a drop down, I don't believe there's an annotation.
Under the Components section on the wiki here
It looks like there are a few solutions for DropDowns. I will take a look at them. The second one looks the simplest, but the first is much more flexible. I like Annotations usage so I like the first two better than the third. The third looks like it requires too much code, having to define the converter.