The most directories aren´t worth to add your site. You will not really get more visitors and beneed this they often use rel nofollow and other technics within htaccess, 302 redirects etc. so your PR will not profit of such an entry.

Also listen as number 100 within a category of a directory doesn´t help your site. Forget it !
The truth is with your entry you will help their site to get higher.

This was an issue in the past but can still happen.

http://www.about-hijacking.org/en/

There are addons available e.g. for FF to show rel nofollow and thats really interesting to find out more about a website or a blog and if its worth to add your comment or site url there.

Very interesting. Thank you for this post Susann. Definitely some things to watch out for and the check my sites within Google periodically.

Thanks, Susann! Please provide names / links of the FF extensions you mentioned.
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Have fun !

Its called SeoQuake:

https://addons.mozilla.org/de/firefox/addon/3036