Google Adwords Keyword Tool

Many keyword software tools use Overture as a source of their keyword lists. But those lists usually don't show plurals, and the keyword phrases usually contain words in the wrong order. You can edit them manually, but this can be a pain when compiling large keyword lists for Google Adwords.

So make sure you definitely use Google's Keyword Tool:

https://adwords.google.com/select/KeywordSandbox

You can get both the plural and singular version of your keywords. It will also help you to find additional related keywords you may not have considered, and give you grammatically correct keyword phrases that should be included. And all this from the largest search engine on the internet.