GNAT is the Ada 2005 version of the multi-language compiler gcc from the Free Software Foundation. It now incorporates almost all new features of Ada 2012.
It is "free" software, which means you can get it freely from various FTP sites (Main site: GnuAda). All sources are available, as well as prepared installation for various machines (PC/Dos/OS2/Windows/Linux, Silicon Graphics, Dec-Alpha, IBM-RS6000, HP-PA, etc.). For MacIntosh, see MacAda. For various versions of BSD, see DragonLace.
And also: JGNAT is a version of GNAT targetted to the Java machine!
Even better!: Martin Carlisle has ported JGNAT to the .NET environment. It is now officially part of Visual-Studio.NET. For more info, go to the A# website...
NEW: A certifiable kernel for critical real-time systems, that conforms to the "Ravenscar profile", is now (freely) available. Information available on the OpenRavenscar site.
Atego (formerly Aonix) offers a free evaluation version of its compiler for Windows. You'll need to register from the download center to get the latest version. There are some limitations compared to the commercial version, but it features a user interface that compares nicely with all Visual-XXX of You-Know-Who. And it allows, even in the free version, to make applets as easily as with Java!
An older version that dœs not require registration can be obtained from the Ada CD-Rom (look under the name Aonix).