Article directories are a dime a dozen. If you are going to do it the hard way, meaning you’d rather submit one at a time and not use the best article syndication service I have ever used, then focus on submitting your articles to the top 21 article directories below.

You won’t benefit from the long tail effect of submitting to the other article directories available, but you will at least hit the big ones with the highest Alexa and Google rankings.
EzineArticles -The big daddy of article directories. If you could only choose one place to submit your articles, this would be a good choice. PR6
Submit Your Article -This started as an article submission service, but has also turned into a high-quality article directory over the years. PR4
Suite101 – Another monstrous article site. PR7
Helium – An up-to-date article site. PR6
ArticleDashboard -Another staple in article directories. PR5
ArticleBase -This article directory has grown to a PR6 with lots of content and praise from article syndication experts.
GoArticles -This one claims to be the largest, but I’m pretty sure #1, #3, and #4 give it a run for it’s money. PR4
WebProNews -You have to be selected to get into this one but it’s well worth the trouble. Old and respected. PR6
SearchWarp -Free to submit and serves a lot of articles. PR5
Buzzle -A strong article directory. PR6
Isnare -Also a submission service, iSnare is another strong article directory at PR6.
American Chronicle -Actually part of a network of article sites. PR6
ArticleCity -Pretty much your standard article directory. PR5
IdeaMarketers -A giant, this is much more than a simple article directory. It is a newsletter builder and management system as well as a “home base” for authors. PR5

ab-articles.com – Avery good website for article submission.
Site Reference -This is a content portal with article submission built-in. PR5
Article Alley -A standard article directory. PR5
TheWhir -Another strong article site. PR6
Articles Factory -Mid-level article site. PR4
SelfGrowth -Won’t find this one in auto-submission software. Must become an “expert” first to then get access to submit articles. PR6
Amazines -Pretty standard. Nothing fancy here. PR3
Article Snatch – It’s pretty popular, but is a “no-follow” directory. PR5