Compression and EQ can very oftenly be overrused and overrated. Granted, compression and EQ are essential tools, but you can tweak them knobs till the cows come home, and your mix will still not sound right.
Stereo separation and positioning plays a huge part in getting a mix right. After messing around with some free stereo plugins, I was amazed at how quickly I was saying to myself, "So that's how they did it!". Of course iZotope Ozone has a great stereo separation plugin, but these are also great to use as well (I often end up using all three) on tracks).
Voxengo MSED is a great plugin for a couple reasons. Instead of separating the frequencies, and scrambling them all over the place, you have a knob for the gain of both the middle and sides. You can add gain to the side, to give a more stereo effect, and leaving the middle signal alone so it sounds good in mono too! Sometimes when dealing with an instrument playing rhythmic chords that you want to be heavily on the sides, but at the same time able to hear clearly mono, multing the instrument onto two tracks and muting the middle on one of the tracks with MSED, as well as some extra tweaking can end up in great results.
MDA Stereo (and a bunch of other MDA effects).
I haven't fully explored MDA's whole bundle, but I tried out MDA Stereo, and I was impressed! MDA Stereo contains haas delay, as well as comb filtering. Mess around with the settings to find out how much this plugin can transform your mixes.
You'll be surprised at how much stereo separation can play into widening a mix, thickening, or cleaning up.
Now, discuss stereo separation plugins (free or not free), and techniques.