Choosing Passive Voice over Active Voice

What are passive and active voice?
Active voice: I am writing a blog entry.
Passive voice: This blog entry is being written by me.
Generally, active voice is easier for readers to parse and understand. However, there are some times when the passive voice is useful. Jakob Nielsen runs a great site on web usability at useit.com and he has an article this week describing how active voice can obscure the meaning of web page in a search engine listing. His example is of an article summary where his sentence starts "Yahoo finance follows all 13 design guidelines..." His studies show search engine listing are scanned by users, and sometimes only the first several words are read. So the listing has more impact when it starts "13 design guidelines are all followed by Yahoo Finance..." - read the article for a more detailed description of how using passive voice allows you to put key ideas at the beginning of headings and lead sentences.