The Google search plugin has "keyword suggest".  Type in "perl" and it
suggests "perl split", "perlico", "perl hash", etc.  Perl is perhaps
not the best example, but I find it's a feature I like a lot.

Most of the other official plugins I've seen don't have this feature
(at least when I last looked).  However there is an easy hack to add
them:

Find your local copy of the plugin and add:

<Url type="application/x-suggestions+json" method="get"
       template="http://suggestqueries.google.com/complete/search?
output=firefox&amp;qu={searchTerms}"/>

Now you can have a Wikipedia plugin that suggests queries as you type
into the search box.  You can do the same for your Perl plugin.  Or
Amazon.

I almost switched to location bar search keywords, but having earlier
added the suggest feature to all my other plugins I found I missed it
too much.   If anyone knows a way to get the suggestions via the
location bar, do please let me know.

One big downside is that I've not been able to find a way to make the
generic Google Suggest feature site specific.  It would be most neat
if I could hack the url above to include "site:cpan.org" and have the
suggestions only come from that site.

That might not be trivial for Google to implement for all sites, but
you'll note that with (not a small) bit of work cpan (or Amazon, BBC,
YouTube etc) could implement their own site-specific version of this
suggest service to contextualize the suggestions in their plugin.  I
still find the service useful, even when it's not site specific.

With the caveat that this page was put together entirely for me, so
your mileage may vary, I have hacked the following plugins to include
the Google Suggest feature.  It would be much better if the standard
plugins did this themselves I think.

http://minty.org/search/

The reason I have my own Google plugin is because of the cruft that
the official Google Plugin introduces into the url.  It doesn't make
for copy&paste friendly.  My version generates the shortest url that
delivers the results.  One down side is that Mozilla probably don't
get the ad revenue because I've removed the magic url bits that say
"This was a search from a Mozilla product".

ps. Perhaps the cpan people could add a line like this to all their
markup, allowing people to add the plugin direct from the site: