The Free MS Windows replacement operating system ReactOS has just released a new version. They have a large and active development team.
ReactOS has been under development for a long long time.
They were in some hot water earlier because some of their code appeared to be line by line dissasembly of some NT kernel code, I think they have replaced all of it.
I wouldn't bother with cross platform testing until they hit the same market penetration as Linux, which I would wager is never.
Targeting ReactOS specifically is a bit too narrow IMO -- perhaps a better focus is to target compatibility with WINE. Because ReactOS shares so many of its usermode DLLs with WINE, targeting WINE should result in the app running just fine on ReactOS.
Of course, there will always be things that WINE can't emulate well (hence the need for ReactOS). In this way, it seems that if something runs in WINE, it will run in ReactOS, whereas the fact that something runs in ReactOS doesn't mean that it will necessarily run in WINE.
Targeting WINE is well documented, perhaps easier to test, and by definition, should make your app compatible with ReactOS as a matter of course. In this way, you're not only gathering the rather large user base of current WINE users, but you're future-proofing yourself for whenever anyone wants to use your app with ReactOS.
In their homepage, at the Tour you can see a partial list of office, tools and games that already run OK (or more or less) at ReactOS. If you subscribe to the newsletter, you'll receive info about much more - for instance, I was quite surprised when I read most SQL Server 2000 tools actually work on ReactOS!! Query Analyzer, OSQL and Books Online work fine, Enterprise Manager and Profiler are buggy and the DBMS won't work at all.
At a former workplace (an all MS shop) we investigated seriously into it as a way to reduce our expenditure in licenses whilst keeping our in-house developed apps. Since it couldn't run MSDE fine, we had to abandon the project - hope in the future this will be solved and my ex-coworkers can push it again.
These announcements might as well be also on their homepage - I couldn't find them after 5 mins. of searching, though. Probably the easiest way to know all these compatibility issues is to join the newsletter, or look for its archives.
I have been tracking this OS' progress for quite some time. I believe it has all the potential to really bring an OSS operating system to the masses for it breaks the "chicken and egg" problem: it has applications and drivers from the very beginning (since it aims to have full ABI compatibility with MS Windows).
Just wait for their first beta, I won't be surprised if they surpass Linux in popularity really soon after that...
Post Edit: Found it! Look at section Support Database, it's the web place to go look for whether a particular piece of hardware of some program works on ReactOS.
Until ReactOS doesn't randomly crash just sitting there within 5 minutes of booting, I won't worry about testing my code on it. Don't get me wrong, I like ReactOS, but it's just not stable enough for any meaningful testing yet!
No, I do not think it is time to start thinking of it as a Windows replacement.
As the site states, it's still in the Alpha stages. More importantly, whos Windows replacement? Yours? Your users? The former is one thing, the latter is categorically a no-go.
As an aside, I'm not really sure who this OS is targetting. It has to be people who rely on Windows software but don't want to pay, because people who simply don't want Windows can use MacOS / Linux, and the support (community or otherwise) for these choices is good.
Moreover, if you use Linux you already have some amounts of Windows software support via Wine.
Back to people who rely on Windows software but don't want to pay. If they are home users they can just simply pirate it, if they are large business users they already have support contracts and trained people etc. It's hard enough for large businesses to be OK to update to new versions of Windows, let alone an open source replacement.
So I suppose that leaves small businesses who don't want to obtain illegal copies of MS software, can't afford the OS licences and rely on software that only runs on Windows and has bad of non-existent Wine compatibility.