Looks like a new version of the Android Market is being silently pushed out to devices running Froyo, complete with a new tab to find related applications. Yours may say "Similar" as it does on the Droid X in the shot to the left, or it may say "Related" as it does on the Evo 4G to the right. For all intents and purposes, it appears to be the same thing with a different label.
While we can't see behind the curtain, it's pretty certain that there's more on the way, either with Gingerbread or as an addition to existing versions of Android. (And, yes, this looks like what we saw in that Gingerbread video earlier today.)
Anyone else out there seeing Market changes? Sing out in the forums! Thanks, Andy, Javier, and Josh for the tips!