Hi,
There’s a BeOS app. -called UniversalScroller- that allows users to set their mouseclick preferences.
Handy for those who prefer to have a left click function as a left double click, for example to reduce the risk of getting RSI.
In Windows there’s an option within the OS itself (and has been there since at least Windows 98, AFAIK).
Beside the fact that UniversalScroller doesn’t function in Haiku now, I think it would be a good idea to have Haiku provide (some of) UniversalScroller’s ‘click options’ in its preferences section.
Cheers,
Meanwhile
Since there are -on this website- intelligent discussions going on about usability, and seeing that nobody has responded to this thread yet:
Justified concerns I’ve read on haiku-os.org about ‘where do you click’ and ‘how many (sub)menu’s should you use’ IMHO fall into the same usability category as my concern introduced here: ‘how often do you have to click’.
It may be that since many/most are so used to the practice of doubleclicking that this isn’t seen as something that needs to be extended with an alternative, but when you think about it there’s really no need to click on a folder twice to open it, similar to the lack of need to ring a doorbell twice to be noticed.
Users of Windows XP can always try how it feels to have reduced their amount of mouseclicks by up to 50%:
Start menu/Control panel/Folder options/Click items as follows…select ‘Single click to open an item (point to select)’.
But as said, it can be experienced in BeOS also:
Download Universalscroller from http://bebits.com/app/1359
and set it up to have your left mouse click function as a left double click.
Cheers,
Meanwhile
I see your single click suggestion and raise you a simulated middle-click by a double-click (left or right mouse button, I don’t care). Not having a 3rd mouse button on my laptop leads to irritating finger gymnastics…
Well, thanks for the nice gesture 
Today I found out what’s very likely the UniversalScroller dev’s real name and his -working- email address, then mailed him, asking to have a look at Haiku and perhaps even make his app work in it…since my request here may have fallen on deaf ears.
Maybe you should try the Glasselevator Mailinglist instead. The main coders seldomly read the forums AFAIK. Maybe it’s even worth reposting the suggestion after R1 is out and the dust has settled a bit. I can imagine that by that time many small request will be forgotten. Mayhaps even a deserving little job to keep track of smallish requests and posting them, all collected, when the time comes. Meanwhile… HAND! 
HAND! Good idea, I’ll do that. (*crosses fingers)
Short Update: Posted on two mailinglists; resulting in a bug report.
http://dev.haiku-os.org/ticket/1298