I want something that is going to help make my mixes louder while retaining a full frequency spectrum and without compressing the mix too much. And without that goddamn scratchy clipping or sudden volume dives.

What you want is a mastering engineer, there is no magic plug-in that can do that. You need a good listening space, really nice monitors, a few mastering plugins, and most importantly mastering knowledge, to accomplish what you want.

The cheapest plug-in I can think of that can help you is, izotope ozone 4, which is available for $250, in the following formats, RTAS/ AudioSuite, VST, MAS, Audio Unit, DirectX. I have been told by a few mastering engineers that this is the best plug-in for the value.

It is interesting you should bump this topic, as I was just thinking about it the other day. Since this topic's creation I have tried demo versions of a few different programs, the Ozone was one of them. I tried PSP VintageWarmer 2, Yohng W1 (a Waves L1 copy), and Ozone 4. I spent a good amount of time with them all and got them tweaked into perfection using a number of different source materials spreading different genres of music and made clips of the resulting sound. The Ozone software was the least impressive of them all, and the most expensive. It's not that the program didn't work, it just didn't do it for me. The W1 is free and to me sounded very very good for a free plug, which is obvious because it was based on the Waves L1 which is pretty popular, it sounded very transparent. The PSP VintageWarmer was the biggest win of the bunch. I found great uses for this plug as a drum bus saturation plug as well as using it as the mastering limiter on the final mix. It seemed to have a warming personality to it that just tickled my ears and made my mixes sound better not just louder. I am keeping W1, I haven't yet purchased the PSP but plan to do so in the next few weeks, and the Ozone is off my list.

If you get the chance maybe you could even try the plugs, they all have free demos.

cool, i'll check those out. I still think ozone 4 is a good plug-in, but its true that you can only use what works for you.

There is a way to get world class mastering done in the box. Having the monitoring to do it well, that's another story.

budget mastering can be done well,
as others mentioned, you need to hire engineer who knows his job;
if you don't have experience, acoustically treated room, great monitoring, amp, dac... - and trained ears - you will make mistakes;
as with ozone - it's not bad sounding unit, just it's easy to make it sound bad if you don't know what to do
mastering audio is not about slamming limiter on the master bus and call it a day,
quality control and fresh pair of experienced ears are priceless!

good luck