<p>There is a <a href="http://meta.stackexchange.com/q/5999/155320">question</a> over at meta.stackoverflow that can answer this for you.</p>

<p>Bear in mind (as Jeff says in the comments) you'll need to do this for every site you have an account on.</p>

<p>I believe this deletes the account, but <em>not</em> the posts. You've licensed them to the site under CC-Wiki. Your username will be obscured.</p>

<blockquote>
  <p>If your account has never voted or posted a question or answer you
  should see a self-delete option on your user page.</p>
  
  <p>However, if your account <em>has</em> voted or posted, do the following:</p>
  
  <ol>
  <li><p><strong>Edit the "about me" field in your profile to say "please delete me"</strong> so we can confirm your ownership of the account.</p></li>
  <li><p><strong>Obtain your user page URL or numeric user ID</strong>. This is linked from the top of every page of the site when you are logged in.</p></li>
  <li><p><strong>Email the support address linked at the bottom of every web page</strong> on the site. Where it says "contact us", click that. In your
  email, you <strong>MUST</strong> include your user page URL or user ID so we can
  find your account.</p></li>
  </ol>
  
  <p>Also of note, we automatically cull old inactive accounts as
  documented here:</p>
  
  <p><a href="http://blog.stackoverflow.com/2009/02/when-is-an-account-abandoned/">http://blog.stackoverflow.com/2009/02/when-is-an-account-abandoned/</a></p>
</blockquote>