One of the best ways to truly experience Paris is by walking her storied streets.  There is no better walking city in the world.  Most of her most famous sites are located within or near the city center.  There are numerous tour companies that can cater to what you'd like to see.  Most of the below tour companies (listed in alphabetical order) offer very practical, and thorough guided walks.

Aeon Tours: offers year-round tours including, a Paris Essentials Tour, a Classic Marais Tour, a tour of the Latin Quarter and a Private Tour. They have a nice, free Guide to Paris on their site as well. 

Classic Walks of Paris offers six walking tours: the Classic Walk, the Montmartre walk, the French Revolution walk, the World War II walk, the Latin Quarter walk and the Da Vinci Code walk.  All tours are conducted in English.  Tours are generally 2 hours in length and offer a mix of history along with interesting anecdotes about Paris and its sites.  The guides also offer their personal experiences of life in Paris which is quite interesting as well.

Paris-Walking-Tours.Com offers private guided walking tours, in English by native English speakers, such as a Paris Introduction Tour, a Marais Tour, a tour of the Parisian Markets, a Latin Quarter Tour, a Montmartre Tour,  and others as well. Insights into history, art and everyday Parisian life.

Paris Walks: offers various tours, depending on the time of year.  Their Summer tours include stops at Montmarte, the Marais, a Private Tour, and tours of the Left Bank.