Here is what I do:
- Remove all cookies
- Login into wikipedia.org
- Logout from wikipedia.org
These steps leave dozens of cookies for various wikimedia sites, even for ones that I didn't use in the session, like wikinews.org, wikitravel.org, wiktionary.org.
Since wikipedia isn't supposed to do any kinds of clandestine user tracking, wikipedia shouldn't leave cookies.
Please delete all cookies when user logs out.