This EQ has the best "autogain" of any EQ i've tried, making it excellent for testing out how different frequency boosts or cuts affect the overall tonal balance. Even if you're planning on using other EQs, you could play around with this one first to experiment with which frequencies are the most beneficial to affect.
The name says it all, this free EQ plugin is slick, and sounds really nice for mastering. You have control of low, mid, high frequencies. Plus, you can choose from American, British, German, and Soviet EQing styles!