The degree to which word stemming [searchengineworld.com] is used varies greatly from search engine to search engine. The particular word or phrase also can play a big role in whether or not word stemming is used.
Out of all the major engines, Excite seems to make the strongest use of word stemming. If for example, you conduct a search for the word boat you will get the same basic set of results as you would if you search for boats. However, using the same process with the words kayak produce two completely different set of results.
You will also get different results when you use variations of the word boat combined with an additional word. (i.e.used boat vs. used boats)
MSN, uses word stemming with it's featured and directory results, but not with the Inktomi results. (you can enable word stemming for Ink from the advanced search page, but the default setting is disabled).
AltaVista, Google and Fast don't even come close to producing similar results when you search on stemmed variations of a word.
As Tedster pointed out, the spellweb tool seems to be simply counting the number of pages in a particular database that contain each variation of the particular phrase. Using a tool like that to try and determine which variation you should target isn't a good idea because, quite often, the version that gets searched on the most will have fewer competing pages.
A good example would be the one Tedster used. Spellweb shows twice as many competing pages for the term cell phone as it does for cell phones but cell phones is the term that is searched on the most.
With other phrase combinations, the results will be the opposite.
The bottom line is that generalized statistics about singular vs. plural are completely useless when it comes to determining what the best strategy is for your particular niche. What is important is finding out what is the most common way people searching for your keyword phrases search. Once you have a handle on that, you can then determine whether or not you may see any benefit from the use of word stemming.