We are a small group who plan to set up a web-site on trade of
tropical agro-commodities.  A free service primarily for users in
producing countries.  The web-site would include some kind of Q&A
service with response from a panel with 5-10 experts. Some paid,
others volunteers.

111. Please give a couple of examples of good disclaimers from Q&A
services (in addition to the short and precise one at the end of
Google-Answers) ?

222. Please give some guidance on the value (enforceability) of
disclaimers. There might be some references to disclaimers in court
cases and arbitrations.
  
333. On some web-sites the user has to click and confirm that he/she
has read and understood the disclaimer. On others it is just a text
somewhere in a corner.  Is there any difference in the
value/enforceability of such disclaimers ?

I can offer some general guidance on questions 222 and 333.  Internet
law has evolved to follow fairly closely with general principles of
contract law in terms of formation of the contract--has there been an
offer, acceptance and consideration (a bargained for exchange of
value)?  Basically, if you have a full and fair opportunity to review
an online disclaimer or license, you willingly click on "I Agree" or
otherwise fully manifest you assent, and you thereafter are permitted
access to the website, most cases have ruled that that is a binding
contractual arrangement.  At the other end of the spectrum, if there's
some legal boilerplate buried three or four links deep that attempts
to bind the user just because they use the site, that would likely
fail the "offer, acceptance and consideration" test.  Thus, it is
clearly preferable to force an end user to click on "I Accept" if you
wish to enforce an online contract?which, of course, a disclaimer is. 
Once you can prove they clicked, it becomes easier to enforce the
agreement, subject to standard defenses to contract formation:
unconscionability, duress, adhesion, etc.  Basically these are all
terms that mean "it isn?t fair to enforce the contract against me even
if I clicked."  There are a number of cases involving AOL's venue
clause in its online terms of service.  These cases deal with the
online AOL contract that subscribers must agree to which says that if
you are going to sue AOL you must do so in AOL's home jurisdiction. 
Some courts have enforced the clause, others have said it is not fair
to make someone with a valid beef against AOL sue them in AOL's back
yard.