I've had a lot of success with Craiglist for posting my rentals, and nearly everything related to being a Landlord (finding landscapers, contractors, etc).  I've also had good luck with RentClicks.com, $59/mo. for a featured property ad.  Was able to rent my Vegas property in first week to quality tenants.

Some LLs swear by the internet listings (craigslist, rent dot com, house dot com, etc.).  They usually work well in metro areas where lots of people are used to using their computer for everything.  They don't work well in rural areas or small towns.  In areas like that (where I live and rent) I end up using several strategies.  The local paper of course, one of those local advertising supplements that comes in a bag on the weekend, yard and window signs work if the street is well travelled.  If the street is busy add balloons, streamers, anything that moves to attract attention.  Colorful fliers work especially well in small towns, posted at groceries, banks, laundromats, neighborhood bulletin boards, senior centers, etc.  (Be sure to put tear-offs on them for phone numbers.)  Other current tenants may be able to locate another if you give them an incentive  ($50 off their rent if new tenant signs and pays on time for 3 months.)  Also, I send fliers to local employers' HR people, armed forces recruiters, and try to get friends or relatives to post it in local employee newsletters.