Which feed directories or ping services are most useful for blog promotion

I would suggest using Pingler or Pingomatic as a free ping service when content is added to your site that you want indexed fast.

There are many ping services to choose from and there are notations in their services about possible over-pinging penalties.  Google doesn’t give a straight answer about over-pinging that I can find, so a bit of caution recommended.

Over-Pinging: Pingler in their knowledge base reports this question and answer: Is it possible to be over-pinging?

Posted by Andre Dedeyan on 07 June 2011 05:21 AM

By default some platforms such as WordPress include their own ping facility, where every post you make is automatically pinged to various resources.

We advise our members and customers to switch off the standard or default ping settings on WordPress or other platforms when using our Pingler service.  This also true when it comes to using other ping services at the same time as Pingler.com

We cannot comment on how the ping resources will react to ‘over-pinging’ but we strongly suggest that you refrain from pinging using other services alongside to Pingler to avoid any detrimental situation as a consequence.