There's a few applications that would help make Linux more of a mainstream OS, but don't expect to see them ported to Linux anytime soon. One of the least discussed in this fashion? Adobe Photoshop.
Yes, I know I've said before that in my purview Linux doesn't need mainstream success to be "succesful", but this is one of those canards that gets waved under my nose often enough that it needs to be addressed at least once.
Why Photoshop? For one, apart from Microsoft Office, it's one of the most broadly used programs in the whole of the computer world, both Mac and PC. Everyone either wants Photoshop or "a program like Photoshop." And in many cases, they don't have the luxury of choosing: they're in a graphic-arts or design job where Photoshop is mandatory, not optional. By far the most overriding reason is support for CMYK colorspaces (you can't do proper graphics work for print without CMYK support). Lack of proper CMYK support is one of the biggest reasons why GIMP, the open-source Photoshop-like app for Linux, hasn't been able to displace Photoshop in a professional context.
And why no Linux-specific version of Photoshop? First, and most likely, Adobe probably believes there just isn't a market for Photoshop on Linux -- yet -- especially since the perceived size of that market isn't even a fraction of its total sales, whether for Mac or Windows.
There's also the question of commercial (read: closed-source) application support on Linux, a topic which deserves its own post but which can be summed up this way: Closed-source apps generally only get supported on a couple of distributions at a time -- Red Hat and SuSE are two of the biggest, although Ubuntu is turning up more and more -- since the effort involved for more than a couple of distros is more than many software companies want to take on.
(This is where I agree at least in part with Alex Wolfe about there being too many distros -- too many for the software makers, but that still means a plurality of choices for the users.)
What's ironic is that a while back, Adobe had an IRIX version of Photoshop available for a number of Silicon Graphics computers. I played with an SGI O2 workstation that had it running, and it operated exactly like its Windows counterpart. Surely it wouldn't be difficult to take the work done for the IRIX version and apply that to a Linux edition? Probably not -- programming for IRIX, Linux, and the Mac OS are almost certainly as unalike as it gets in many ways.
Finally, there's the problem of third-party add-ons. Photoshop has a giant library of plug-ins, and many Photoshop users are married to their plug-in collections. Said plug-ins would not work on Linux, unless a) they were rewritten from the ground up (not terribly likely) or b) the Linux edition of PS had, say, some kind of back-end into Wine that allowed the plug-in to run correctly. There's always the possibility of running the Windows edition of Photoshop in an emulated Windows session or in Wine, but that sort of defeats the point.
So if Adobe ever bothers to offer Photoshop for Linux, I suspect it's going to be for very specific breeds of Linux, and not Linux generically. I'm dead certain Adobe is not about to make Photoshop into an open-source product; they're going to be as stalwart about this as Microsoft is about Office. But again, it's a question of how much Adobe feels it's likely to get back for that effort -- which, at this point, is probably not a lot at all.
[Postscript: There is indeed an add-on package for GIMP that provides CMYK support -- Separate -- but it would still be nice to have CMYK supported in GIMP from the ground up.]