So today I received an email about Plugin Alliance having a sale on Fiedler Audio Stage
Coupon: FIE-STA-2999
What are other people’s go to plugins for Stereo Widening or other stage effects like this?
I’ve pretty much only used WA Productions Pumper.
The more I look at this the more I’m reminded I need to pick up Boz’s Mongoose.
That looks like it does just about everything you can imagine! My absolute “go-to” in this department is @bozmillar Mongoose. Mono-ize your lows, widen your highs. Straightforward, no nonsense and beautiful in its simplicity! Boz also has a FREE stereo widener too.
I also use the JST SideWidener (free with Computer Music mag) which Boz also happened to program. It gives you some “flavor” with your widening…
Somehow I missed your second post and was just replying to your first! Seriously! If you’re patient, both of these go on sale occasionally. 
I’ve never really been a fan of “spacial”-type plugins.
About the only thing I’ve consistently used is the mid-side function of T-Racks old faithful Classic Compressor.
It doesn’t suit everything though…
I tried monoing the low end a few times and really disliked the result, tbh.
Call me a crusty old curmudgeon, but I think the most effective stereo widening tactics are judicious use of simple eq and panning. Some of the most incredible stereo recordings that I admire (eg late 70s & early 80s Steely Dan) were done before all this stuff was in common useage.
That said, Tchad Blake reputedly uses a cheap old analogue box for stereo widening, and his mixes are consistently amazing…but he’s Tchad Blake…
I tend to agree - the best way to make things sound wider is to turn up hard panned things. And also to get difference between the left and right side of the arrangement, so your brain doesn’t join them up as much in the middle. So, hard panned guitars where you’ve used different rigs for each side is one thing, but having a guitar hard left and the mating call of a mongoose hard right sounds way wider. 
The trick is to do it without it sounding disjointed or distracting.
That’s not to say that stereo widening doesn’t have its place, but usually subtle and limited application is the order of the day in my book.