In Eclipse, when working with Java, I'm using the excellent AnyEdit Tools plugin by Andrey Loskutov. The most important feature for me is to be able to compare a selection in an editor with the clipboard content. This can be used, for example, to compare two blocks of code for similarity, to help assess how easy it is to refactor the blocks. Consider this code:
Now, I would like to select the first block, copy-paste it, select the second block, and compare the selection with the clipboard content to get a diff between the blocks only. I'd love to see that in SQL developer. Of course, there are other useful features in the AnyEdit plugin, but this one is my favourite.