http://www.coloproperty.com is directly tied into the Northern Colorado MLS and has very few scams.
Craigslist is full of scams and you have to be very careful when looking there and NEVER send any money to anyone for anything. :)
Hope this helps!
For Boulder County rentals I generally send people to the following website search engines:
Trulia.com (of course)
Let us know which of these helps the most!
Prudential Real Estate of the Rockies