FinditClassifieds.com says it has been online since 1997 and that it has many anti-spam and anti-fraud features and that it is more family friendly than many other sites since it does not have personals or any sex related ads.
I would ad it to my list of free classifieds sites for sure.
Best bet is to have a property management company or real estate broker rent the house for you. They are very good at screening prospective tenants and obtaining rental & credit reports. I don't work in the Waldorf area but have several collegues who do. If you want a referral give me a call.