Yeah, I remember that one. Had a Windows copy installed for a while before 2.5 
One… 2.71
I don’t see where you really need any other than the newest release.
Always only the latest official release for real work, sometimes a version in-between, usually Release Candidates to check out some new features
2.66 was the first version I downloaded. I really wanted to make a Redwall animation. I never succeeded, but succeeded in different things!
2.69 and 2.71, but 2.69 is only left there because my PC is old and sometimes I need to check things are not due to odd graphics card behaviour.
I only run the latest stable version.
That said, sometimes I download (or use already unzipped copy, since I always download zip) of previous version when a tutorial uses that version.
I keep the official release and the latest SVN. I download the SVN off Buildbot every couple of days.
Official release and ofcourse the best version - colored wireframe (now 2.70).
2.69 because if you take anything created in that and earlier versions into 2.70 and above look out. I’m not a Techno so I have no idea what happens but it’s as if the file is immediately contaminated. And, even if you renamed the file it’s like somehow it comes right back to the original which was never opened in 2.70 / 2.71. Someone mentioned staying with the version the project was created in that seems to be a outstanding idea especially now.
Just one: latest dev version.
I like some excitement in my life. :evilgrin:
I keep a copy of Blender 2.69 around for importing from MakeHuman Alpha 7 until the current MH releases reinstate a facial rig. I also occasionally download releases from graphicall or the buildbot to check out new development. Other than that I pretty much exclusively work with the latest stable build from the Blender homepage whatever that is at the moment (2.71 at the time of this writing).
So, at any given time I typically have three versions of Blender installed.
I switch between the buildbot build and the latest stable release. I’ve been using the buildbot builds lately because I can’t function without pie menu’s.