Update: Hotmail now offers POP access to all users. Connect here.
Though not offered by all free email providers (i.e. Hotmail), Google offers external email reading via the POP3 protocol as part of the Gmail service.
Setting up your iPhone to read your Gmail is fairly simple.
First, insure that POP is enabled in your Gmail account. Go into your Gmail settings and click on the "Forwarding and POP" tab. Once inside, enable POP access (for either new mail or all mail, it's up to you).