Opera Mobile
Fantastically useful and occasionally hilarious, if your idea of humour includes jokes expressed entirely in failed CSS.
It was really useful to use when I was building a responsive layout, as it does add a few bells and whistles to check if something will work on a mobile. But if you just resize the browser window (and sometimes, zoom) it should also show the responsiveness, and that works for layout. When I saw it change just on resizing the window I had to boot up the emulator as a sanity check, and yes it really does do that. I mean, wow.
And then just remember that pixel density also fucks over your media queries as you try desperately to see everything on a retina display...
Yeah, yeah. At least there's the work iPad to test this stuff on.