LiveCode is a multi-platform development tool that lets you use one scripting language to target Macintosh, Windows, Linux, iOS, Android and web servers. Two apps on this site are being delivered to 4 different platforms (iOS, Android, Mac and Windows) all from a single code base. LiveCode is Open Source so the Community version is free. There are also paid versions which provide some additional features that commercial developers find attractive.
Find downloads for all versions of LiveCode here.