What kind of directories are we talking about here?
From my experience, FFA lists and random directories (the kind that get included in software programs that submit your website to 5 million + sites) usually result in a net gain of NOT A SINGLE extra visitor per day, so don't waste your time.
And as far as I know for SEO purposes, search engines don't put much stock in directory submissions either.
With that said, if you want to submit your website to every place possible and just get listed on the search engines there are plenty of websites that do that for you and their going rate is usually between $19 and $29 for the service. I have used these services in the past and my website has always appeared in Yahoo, MSN, and Google searches soon afterwards.
I think they are all pretty much the same and use similar lists of search engines and "directories". Quite frankly I wouldn't be surprised if they all used the same exact back end software. With that in mind, I don't think it really matters which service you use, any will do.