Yes, but unfortunately each one has significant drawbacks.
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Kynetx
supports IE, Firefox, Safari, and Chrome, but extensions are dependent on the Kynetx extension to work. In addition, Kynetx apps are built using an odd, proprietary Kynetx Rules Language. There is no cost to use the Kynetx platform. UPDATE: Kynetx has shut down -
WebMynd
supports IE, Firefox, Safari, Chrome, and is in private beta. From what I have been able to determine, WebMynd is a very small San Francisco-based startup that is still getting off the ground. Their library is standalone and provides a Javascript API, and they use a monthly billing model. UPDATE: company pivoted, now for mobile, trigger.io -
Crossrider
supports IE, Firefox, Chrome and Safari. Recently out of beta (as of June 2012), Crossrider is free and offers a jQuery-based development experience. Publishing methods for your extensions include an embeddable website widget, download links, and a customized windows installer. However, CrossRider does not allow you to self-host your plugin files. UPDATE: As of 27.09.2016, CrossRider is terminated and it will be available only until 27.10.2016. -
KangoExtensions
supports IE, Firefox, Chrome, Safari, Opera. Seems very new and bleeding edge, like Crossrider, also looks promising.UPDATE: No updates in over two years.
IE and Firefox seem to be the hardest to develop for, although Mozilla JetPack aims to greatly simplify Firefox extension development.