A couple people have mentioned that they installed ProcessWire, logged into the admin, logged out, and couldn't find where to login again. The default login URL to ProcessWire is:
yoursite.com/processwire/
You can also change the login URL by editing the admin page (Pages > Admin) click on the "settings" tab and change the URL name to something else. Just don't forget what you name it otherwise neither you nor I will be able to figure out what the admin login URL is.