First off, there's this GORGEOUS coat from McCall's. Not only would this be perfect for a steampunk outfit, it's amazing for every day. If I lived somewhere where I needed a coat every day, I'd be all over this.
This is a vintage McCall's pattern which is obviously not your typical steampunk period, but I think could be made to work well with some small adjustments. (Changing the shawl collar and adding double-breasted buttons, for example.) Plus the cape is detachable.
McCall's 7254 is actually a sweater pattern, but this variation feels like it has potential. Not sure if it would work in a non-stretch fabric of course, Sizing would probably have to be adjusted.
Vogue has this super cute children's jacket. I think this would work great for steampunk for either a girl or a boy. So if you have little ones to costume, take note!
In all the excitement over the new Simplicity costume patterns I missed this awesome coat. Cape and epaulets? Would easily work for steampunk.
Here's something else I overlooked on the Simplicity site. Capes are apparently "In" this season, and I'm loving it. I love the military details on this one. I may have to make myself one.