Cokesbury offers a number of resources for preaching on the web and on CDRom, including iPreach and the Sermon Connection: www.cokesbury.com. 

College of Preachers, a Washington, D.C.-based center for preaching workshops and resources: www.collegeofpreachers.org

Homiletics Online, emphasizes contemporary resources for sermon ideas and development www.homileticsonline.com

Lectionary Homiletics.  A well-regarded and highly used print and web-based resource for lectionary preaching.  http://www.lectionary.com/

Preaching On-Line, an extension of Preaching Magazine.  www.preaching.com