I do a decent Kermit, and as an extension of that, a decent Ernie. My voices tend to skew towards Steve Whitmire's versions.
I once did a Rowlf voice. I worked on it enough to sing in character at a family reunion, but I've pretty much stopped doing it, so it sounds awful now.
I also do a passable Lew Zealand and Statler, as voiced by Jerry Nelson.
I'm also shockingly good at sounding like Janice, which is odd because I've never really had the need or even wanted to do her voice. Just comes naturally for some reason.
Most recently I've been working on Sam the Eagle, but it's far from perfect at the moment.