The 'Classic' AmigaOS line came to an end with the release of AmigaOS 3.9 in 2000. Since then, there are three main projects to replace the AmigaOS with new and updated versions of the Amiga operating system, running on more modern hardware. Because of this, however, 'New' Amigas are not fully compatible with 'Classic' Amigas. As of early 2010, there are three main projects to be aware of:
This is the 'official' successor to the AmigaOS 3.x line. The current version is AmigaOS 4.1 (released in June 2009) and it runs on certain PowerPC-based computers, namely AmigaOne machines and Pegasos systems (both now discontinued) as well as SAM 440 machines (still in production in late 2009). Its predecessor, AmigaOS 4.0, also runs on certain 'Classic' Amigas with a PowerPC accelerator card.
It was developed by Hyperion, licensed from Amiga Inc (current Amiga copyright holders) - but was the subject of a lengthy court battle between the two companies, eventually settled in Hyperion's favour in September 2009.
AmigaOS 4 can run applications written for 'Classic' Amigas as long as those applications are written in a 'standard' way and don't hit the custom chips directly. This excludes almost all games written for 'Classic' Amigas, which have to be emulated (i.e. in the same way you can emulate 'Classic' Amigas on a Windows PC).
MorphOS is an alternative Amiga-like OS, which like AmigaOS 4 runs on PowerPC-based machines. Like that OS, the latest version of MorphOS (2.4, released October 2009) runs on 'Classic' Amigas with PowerPC accelerators and Pegasos machines. It does not run on AmigaOne systems, but it is compatible with EFIKA machines (still in production as of late 2009), and the Mac Mini G4.
Many of the developers of MorphOS, Pegasos and EFIKA were former employees of phase5, a company that used to manufacture accelerator cards for 'Classic' Amigas, (including the Blizzard and Cyberstorm lines).
MorphOS compatibility with 'Classic' Amiga applications is similar to AmigaOS 4 (see above).
AROS used to stand for 'Amiga Research Operating System' - after legal threats from Amiga Inc, it now stands for 'AROS Research Operating System'. It runs on standard x86 PC systems (like Windows PCs), although only with certain configurations. It is also available in a virtualised environment called Icaros - this version will run on any modern PC. It has also been ported to some PowerPC-based machines, including SAM 440 and EFIKA.
It is open source software, and so is freely distributable. Despite having been in development since 1995, as of late 2009 it still isn't quite a finished '1.0' product - although it is quite close and is generally usable and being actively developed.
AROS is designed to have the look and feel of 'Classic' AmigaOS, and indeed its 'nuts and bolts' work in exactly the same way. However, unlike MorphOS and AmigaOS, it is only 'source' compatible with 'Classic' Amiga applications. In other words, it is very easy for a developer to port their application from 'Classic' AmigaOS to AROS, but the program won't run 'as is'. However, it does include an emulator for 'Classic' Amigas (MorphOS or AmigaOS 4 also require the use of an emulator when it comes to games and some other software, so the difference is not so great).