Here's an alternative to the various toolkits and HTML hacks people are posting: How to do a cross platform GUI? DON'T.
As a user I expect applications to stylistically conform well to the native UI rules. I want a Mac application to look and feel like a Mac application, and an Android app to look and feel like an Android app. When an application uses one of these "cross-platform toolkits" I can tell instantly by using the application for a few seconds. Odd things around scrolling, menus, mouse hovering, how things re-draw when you resize the window, etc. They make your toolkit-built app stick out like a sore thumb. Yuck! Some toolkits are better than others, but I'm pretty sure I can always tell!
Sacrifice a little to do your users a favor: Build all the business logic cross-platform, with an API layer callable by whatever the best native language is, and do the UI portions of your application with the platform-recommended native tools.
As a user I expect applications to stylistically conform well to the native UI rules. I want a Mac application to look and feel like a Mac application, and an Android app to look and feel like an Android app. When an application uses one of these "cross-platform toolkits" I can tell instantly by using the application for a few seconds. Odd things around scrolling, menus, mouse hovering, how things re-draw when you resize the window, etc. They make your toolkit-built app stick out like a sore thumb. Yuck! Some toolkits are better than others, but I'm pretty sure I can always tell!
Sacrifice a little to do your users a favor: Build all the business logic cross-platform, with an API layer callable by whatever the best native language is, and do the UI portions of your application with the platform-recommended native tools.