For freeware the GIMP and OpenOffice are pretty decent software. OpenOffice even can create PDF files whereis Microsoft Office does not have that ability (unless Office 2007 does). Free software like this makes Linux more acceptable and you can get GIMP and OpenOffice for the Mac and Windows. When you consider what you can get for free it makes me wonder why Microsoft Office and Photoshop cost so much.
However, I personally do not care too much for open source software. I have bad memories of finding a possible backdoor in Firefox. I owned a Windows computer a while back that had all kinds of free software-OpenOffice, free version of ZoneAlarm, free AVG antivirus, free antispyware software, GIMP, Firefox, Thunderbird, etc.
I don't have a lot of experience with Linux but for a while I toyed with the idea of getting a custom built computer that could run Linux and using free software on it. But the problem is that if you work on photo editing you somehow have to locate scanners, photo printers, etc. that are truly compatible with Linux.
Probably the best computer for computer graphics is the Mac although a Windows computer can get the job done too. I suppose a Linux computer could get the job done if you were doing digital photography, did not need much in the way of a scanner, and could somehow find a compatible photo printer. Personally I have pretty much given up on Linux even for a server computer. OpenBSD is better as a server and Apple Computers and computers running Winners are hard to beat as desktop computers. And now they are finding all kinds of exploits that can be used against Linux operating systems.
I don't know if anybody will ever truly replace Photoshop. Actually, Adobe Software itself may have found a possible replacement with Lightroom, their own product. Lightroom is cheaper.