I often to reference an analyzer to see what I need to address. It often shows me what frequencies I can scoop out without losing to much of the impact of the sound. Say (for instance) that a certain sound was amazing, but once it was in the mix, it was drowning out a bunch of other sounds. You can use the RTA to find where the fundamental sound is and trim away a lot of excess. It can really help you tighten your mix without too much trouble
I often use it to show me where my trouble spots are. If I have a hum or ring I can't quite find, I'll jump to my RTA.
There's one built into Logic (if you have it). I also have a few others here and there, but most came packed with some bundle. Nothing that is absolutely free off the top of my head, but then again, I'm not really up on free plugins.
Yeah, some people say it's cheating, but who cares? It's a tool that I find very useful, so I use it. There are no rules in Sound Design except that "if it sounds good, use it". So if using an RTA makes your work better, use one!