I now use four operating systems: Windows 7 at work, Ubuntu 12.04 at home, iOS on iPad in many locations, and my Android smartphone.
A great deal of my work has moved to the cloud, where I can gain access to it from all these devices. Google has my Calendar, Tasks, and Contacts. the calendar syncs to my Exchange server at work (although I understand that Google doesn't offer that program anymore).
Many of my working files are on Dropbox, where I work with them from Office suites on all the individual platforms (Microsoft Office, LibreOffice, Polaris Office, QuickOffice). There are difference between them, but all of them do the modest work I need from them (mostly editing small word processing, spreadsheet, and presentation files). I keep a lot of little files on Evernote, mostly haiku.
I use a fair number of cross-platform tools: Notecase Pro for Windows, Linux, and even Android (although it's a little clunky on the phone). It doesn't exist on iOS, although Plaintext is kind of a workalike. Notecase Pro remains my primary work/task management tool, a combination outliner, text database, and journal. I use the Xmind mindmapping program for Windows and Linux, with work-alikes on iOS (iThoughts HD) and Android (Mindjet). I see that SimpleMind, another mind mapper, is now on Windows, iOS (where I use it a lot for talks), and Android (although it's really too small on my phone's screen). I use Kompozer to edit my web page on both Windows and Linux, but not iOS or phone.
I have clients for my email on all platforms. I have eBook reader clients on all platforms (Kindle, Adobe, 3M, DCL).
I have Twitter clients on all platforms, sometimes through a browser, sometimes not. Mostly, I use Chrome, but the browser doesn't really matter that much.
The only other tool I seem to reach for is a single pane outliner. They're just wonderful tools for thinking. I've got tkOutline on Windows and Linux, the marvelous CarbonFin Outliner on iOS, and the Android Outliner. Moving things from one to the other - importing and exporting, synching - gets to be more important all the time.
Bottom line: I'm using more software than I used to, and by preference, it's open source, or cheap. But the focus is more on the file than the tool. Most of the work I do is ephemeral (browsing, email). The few things that aren't need to be stored and backed up where I can access them from anywhere, and in a file format that permits the easiest portability.