The Eclipse scala plugin has a very nice info. One can hover over pretty much anything be it a function definition, val, var, function call etc and get type info. This is im
Type Info is great in IntelliJ; you have to press the key shortcut to see the type of a given variable or function.
If I recall correctly, there's a conflict between Type Info's key shortcut and something else in IntelliJ. If you go to Settings -> Keymap -> Plug-ins -> Scala -> Type Info, double-click it, you'll see the conflicts. I changed mine to Option ⌘ =. (see this answer for more tips if it still doesn't work)
Another shortcut I use quite a lot to quickly look up a definition is ⌘ shift I.