While Wikipedia is often not allowed as a cited source in school research projects, it can be an amazing tool for pre-research, as well as for finding primary source resources that are in the public domain (i.e. okay to use).
Helps you find information faster!
It tells you where the authors got their information, so you can go to those sources.
Text in blue are links, so you can visit them easily.
A list of books on the subject that were also used in creating this entry. Clicking on the ISBN will take you on a search for this book--to purchase, if it's still under copyright, or to read digitally if it's in the public domain.
Provides lists of other related content you might want to investigate.
Links to media (photos, video, text, etc.) that are in the public domain.
An article might also link to books, articles, etc. written by the subject.
Just like the Dewey Decimal Classification, Wikipedia categories its articles by various subjects, and provides that list--you can use it to click to broaden or refine your search.