For JAWS, go to Freedom Scientific and they have a demo for thumb drive. You'll need to be admin on the Windows box in order to install the Video Intercept.
What's cool is you can put the thumb drive on any machine that has the video intercept, portable screen reader. Comes with MAGic for screen mag testing (which you want to do). Yahoo! has a horrid, hard to hear and understand video showing a screen mag user (using the bigger name, ZoomText), still cool to see tho: http://www.youtube.com/watch?v=-VoCvUi4Mrg
MAGic works well with JAWS tho.
Handy Guide to get you started: http://www.freedomscientific.com/doccenter/archives/training/JAWSKeystrokes.htm
-It's a real product used by real people (I'm not comparing this to NVDA or VoiceOver, but to things like FireVox and Fangs).
-As a real product, it has real bugs. For this reason, I'll say you may want to strongly consider testing in many screen readers. If you have a Mac, test in VoiceOver (remember in-page skip links won't let you jump large menus or anything... a known webkit bug tho, you can't do anything about https://bugs.webkit.org/show_bug.cgi?id=17450), get NVDA and test (way easier than getting JAWS and very easy to get started) and if there's a demo of Window-Eyes, HAL or that SystemAccess To Go... try them too.
Screen readers are like browsers, in the bad way (all have bugs). If you suspect your audience is in a particular country or region this can help you decide where to do heavier testing.
-According to Freedom Scientific, the JAWS demo is not for developers to test things in.
-The demo lasts 40 minutes. To get more time, you have to reboot. Or, I'm sure some clever 12-year-old found a way around that.
-The current version is 13. Be aware of major bugs of older versions: as expensive software, and JAWS being quite popular, assume you have users with older versions. I would think version 9 would be reasonable to support. My first version was 7.something, which only worked in IE6 and barely in IE7.
NVDA is great, and it has probably the best ARIA support out there. It's also less-well-known, is developed by just 2 dudes (very cool ones, but just 2), and JAWS has been hacking (and reverse-engineering) windows software for much, much longer and can just do more. And the JAWS voices are better. If NVDA's grate (like Orca's) and you'll be doing a lot of testing, see if you can get some better voices somewhere.
Window-Eyes, from what I hear, while being very popular (maybe #2 after JAWS?) is very behind the times with things like HTML5 and ARIA.
Orca may not be on your Linux system. You can either use your packaging system or, if you're using a newer Gnome version (Orca works primarily with Gnome), you could try pulling from master. http://live.gnome.org/Orca/DownloadInstall
Orca works best on Gecko. They're working on webkit, as Gnome apps are slowly switching over to that engine. But testing in Chrome or Epiphany will disappoint. Use Firefox for testing web pages. Keep in mind that unlike most other screen readers, Orca does not use a virtual buffer. Live changes are live, dawg!
VoiceOver: if you are testing non-English, you have to go get the non-English voices yourself. This sucks. Then you have to set that language in the Accessibility section or something. I don't have a Mac, but we discovered this when a colleague got one. Mac was bought in a Dutch Apple shop. OS came with Dutch settings, everything in Dutch. Except the Accessibility, and VO couldn't speak Dutch. That's horrid. Easiest voice to listen to seemed to be Vicki. Kinda Portal-ish. About half the voices on the Mac are novelty voices and completely worthless
++ to that