I know what you are thinking. "What the Bulbasaur is an Eviolite?" That's what I thought too. Then I read into it a little. This tricky little item increases the defense and special defence of a Pokémon that is not fully evolved by 50%. In some instances, this doesn't mean much. In others, this can make certain Pokémon not only an even match for their fully evolved forms, but surpass them. Let's take a look at some of those instances.
Because Chansey and Blissey's only important stats are HP and Special Defense (because Chansey will be used as a special wall 99.9% of the time,) Chansey makes a great utilizer of the Eviolite. This increases Chansey's Sp. Defense from 105 to 157. This surpasses Blissey's Sp. Defense stat, which sits at 135, and while the evolution to Blissey does come with some additional HP, the difference in base stats is only five. Does this mean that Chansey is now completely, utterly superior to Blissey? No. Because Chansey is holding the Eviolite, it cannot hold any other items. That means no leftovers, no Lum Berry. However, it does mean Chansey is now probably the better overall choice for your Sp. Wall.
What? Porygon2 as a wall? Yuppers. Porygon2's defense stats are pretty decent as it is, sitting at 90 and 95, respectively, but adding the Eviolite boosts those up to 135 and 143. Combine a Porygon2 holding the Eviolite with Toxic, Protect, Rest, and Conversion 2 and you have yourself a mixed wall that is more annoying to your opponent than an episode of Everybody Loves Raymond. (Zing!)
I hate to break it to you, but Scyther might be better than Scizor now. Add an Eviolite and Scyther's defense stats go up to 120 each. This makes Scyther's stat total 580, far greater than Scizor's 500. Eviolite can turn Scyther into a decent sweeper that can also take a hit. However, as mentioned earlier in the blurb about Chansey, the only negative is that you can use any items. A choice banded Scizor is probably still better than a Scyther with an Eviolite.
Gen IV introduced Rhyperior, an often forgotten evolution of Rhydon. Walp, now he will be even easier to forget, because Rhydon with Eviolite has a defense stat of 180. This is another instance like Scyther, where Rhydon can become a sweeper that doesn't mind the feeling of a Stone Edge or two. The problem with Rhydon, which is the same problem that Rhyperior has, is its lack of speed and many weaknesses. With how much Surf is used in competitive play, having a 4x weakness to it is bad news bears.
Murkrow (Dream World)
This one is a little weird. In the upcoming Dream World, Murkrow can have the ability "Prankster." This ability increases the priority of non-damaging moves. Normally, this wouldn't be a big deal, because Murkrow, and its evolution Honchkrow, have laughably low defense stats. With the Eviolite, however, Murkrow's defenses jump to 63, which, although not fantastic, means Murkrow might be able to take a couple of hits if you do it right. For instance, a Murkrow with FeatherDance can lower the opponent's attack two stages before they are able to do anything. The Murkrow will probably survive that attack, and even add in a Toxic and a Rest, possibly allowing you to fight again another day.
So there we have it. If we've learned anything this week, it's that Smogon is going to have a very, very frustrating time sorting through their "Not Fully Evolved" tier to find what ones are worthy of competitive play with the addition of Eviolite.