Why do we worship tulasi?

Tulsi plant can be seen in all the Indian homes, infront or central courtyard of hindu house. Traditionally the tulsi is planted in a special masonry structure known as Tulsi Vrindavan as it is related to their culture.
The lady of the house lights a lamp, waters the plant, worships and take a round of the tulsi vrindavan. The plant is regarded as a "womens deity" and a symbol of ideal wifehood and motherhood. Tuesdays and fridays are considered especially sacred for tulsi worship.
In sanskrit, tulanaa naasti athaiva tulasi-- that which is incomparable is the tulasi. For Indians it is one of the most sacred plant.
As one story goes, tulasi was the devoted wife of shankhachuda, a celestial being. She believed that Lord Krishna tricked her into sinning (sin). So she cursed him to become a stone. Seeing her devotion and commitment to righteousness, the Lord blessed her saying that she would become the worshipped plant, tulasi that would adorn his head. Also that all offerings would be incomplete without the tulsi leaf. This is the reason behind worshipping the tulsi leaf in all our rituals.
The tulasi leaf has great medicinal values and it is used when we get a common cold.
There is a saying in sanskrit :
             Yanmule sarvatirbaani 
                   yannagre sarvadevataa
           Yanmadhye sarvavedaascha
                   tulasi taam namaamyaham (source: book-In India culture why do we...
                            By-Swamini Vimalanand & Radhika Krishnakumar)

Meaning:
                      I bow down to the tulasi,
       at whose base are all the body places,                   at whose top reside all the deities and
      in whose middle are all the vedas.
(Source: In Indian culture Why do we..... Swamini Vimalananda and Radhika Krishnakumar)