The AutoGain plugin is great for leveling bass, vocals or anything else. It is also great for doing what amounts to ducking with gain instead of compression.
http://www.hornetplugins.com/plugins/hornet-autogain/
I just picked up the VU Meter as well because I am very heavy into Nebula these days and I would like my gain staging to "maintain itself" once I get it set up initially. This will save me time during a mix.
http://www.hornetplugins.com/plugins/hornet-vu-meter/
Hopefully others will chime in with experiences based on other plugins. I am sure there are opinions all over the internet if there is something that looks interesting to you.
I have all of my standard plugins that I use and I am not looking for others, otherwise, I would take a look at other offerings here based on my positive experiences so far. HoRNet plugins seem to have an angle to them that others are not doing.
AutoGain has been fantastic for me, and I anticipate that VU Meter will also improve my workflow.