As far as I know, the PL/SQL Developer (at least the current version but I think that at least some previous ones also had that - not sure if it was v.8 or v.9) has a built-in feature of SQL Window to execute statement under cursor (current statement) that does not require the plugin. (PL/SQL Developer did not have that before, eg. version 7, so the plugin was a necessity and appreciated its existence.)
By default (if you did not change your keyboard shortcuts and the AutoSelect preference), the F8 (and "Execute" icons in toolbar) will execute all the statements in the window (unless you select some text, then the selected text gets executed) and Ctrl+F8 will execute only the current statement (again unless some text is selected). The AutoSelect preference, when enabled, currently reverses how the keyboard shortcuts and toolbar buttons work in that matter.
You can redefine both keyboard shortcuts. As far as I remember (I do not have access to the application at the moment), one is bound to "Execute" action of "Session" menu (or some kind of another menu) and the other is called "Execute current statement" or something like that and is located somewhere at the bottom of the key-binding list, probably as a SQL Window related action.