SeaMonkey has this in the status bar! There's a little power plug icon you can click to go offline/online. It will even still happily load pages from cache whil

SeaMonkey has this in the status bar! There's a little power plug icon you can click to go offline/online. It will even still happily load pages from cache while offline (so I can click from this page back to the HN front page and onto articles I've clicked on or comments I've viewed before, and it just loads them as you'd expect). If you click on a link it doesn't have cached, it'll pop up an error screen and prompt you to go back online.