Calibre

If you click your username in the upper right and go to Settings, there's an Import option under Data on the sidebar on the left. It gives you a link to Goodreads' import/export page where you can export a .csv that Bookwyrm can import. I'm in the middle of the import right now, so can't confirm how well it works, but that should get you started.

Edit: 4% in to the import and it's already going off the rails. Literally the second book in my export list got imported completely wrong, with Star Wars: Convergence somehow getting imported as something called Parabola.

Second Edit: Import is done but just about everything is wrong to one extent or another. I was trying to sort everything out, but then it decided that books 1 and 2 of a series were the same book so it wouldn't let me enter separate entries for them (despite me entering distinct ISBNs for each), it just kept telling me that I already had a different edition of the book on my Read shelf. This whole thing needs a bit more time in the oven if I'm going to ditch Goodreads for it. Not worth the hassle at the moment.