Voxengo's SPAN plug-in is a free piece of software designed for spectrum analysis and metering purposes. It's available in a few different formats, including in VST for both PC and Mac operating systems and as an Audio Unit plug-in for Mac. You can download the plug straight from Voxengo's website at no cost to you, which of course is the best thing about this plug-in. It's not at all hard to use, but provides you with a nice collection of tools with a correlation meter, output meter, full frequency spectrum graph, and statistics like RMS, clippings, and max crest factor. It's really got a lot of features, especially for a free plug-in. It's easy to use but if you feel lost just do some research on what everything does as I don't think there's a manual.
I'm currently running the VST version of Voxengo SPAN on my system at home. I've got a Mac and Pro Tools 9, but also am using the FXpansion VST to RTAS plug-in wrapper, otherwise this plug-in would not be compatible with Pro Tools. My computer is a Mac Book Pro lap top with a 2.2 Ghz Intel Core 2 Duo processor and 4 GB of RAM, while my audio interface for Pro Tools is a Digi 002R. I'll only run one instance of this plug-in, usually on the master fader, so it doesn't take up much of my processing power at all...
Voxengo SPAN is the most complete frequency analysis and metering plug-in that I've used as far as the free ones go. It's got a very clean looking interface that's extremely easy to follow and very helpful for mixing. There's no reason not to have it if you have a compatible system, although Pro Tools users without a VST to RTAS wrapper will be at a loss. Definitely do yourself a big favor and get SPAN today.