ScreenSteps Live combines a hosted documentation service with a desktop client for quickly creating screenshot based documentation. It also integrates very nicely with Zendesk. You can see an example here:

http://help.screensteps.com

Or, here is another example site we set up:

http://ipad.screensteps.com

Very simple to use and very effective. You can read a case study here:

http://www.bluemangolearning.com/screensteps/case_studies/se...