Here's another link that lists the top 5 for 2007 (according to their polls):
Top mortgage companies in the United States and Canada
But there's also some interesting articles there too.
Home Loans, Refinancing, New Home Financing: GMAC Mortgage
GMAC is quite popular as well. They purchase and service many mortgages from banks ... and their billings are detailed sufficiently that if you choose to pay more toward principal, you can detail it when you send them a check. They break down the portion of $'s for interest as well as the $'s for principal and confirm the amounts allocated on their next billing statement; you always know the exact amount due in total and the exact YTD totals.