[I think the category would be "tours", but suppose it could be broadly placed under "travel". I didn't find this under your list under Paris, but will confess, I didn't look through the whole list.]
Paris Walks was founded in 1994 by Peter and Oriel Caine, a British couple living in Paris. They have several professional guides who work for them, all of whom are university educated and are writers or teachers or translators, and speak excellent French. There are usually two tours a day, in good weather (less frequent in winter) and often three a day on weekends or holidays. The tours begin (usually) at 10:30 a.m. or 2:30 p.m (except as noted in their schedule which is on their website Currently (August 30, 2012), they cost 12 euro (slightly less for students or seniors). There's a possibility of private tours (see website). The "basic" tours are various parts of Paris: the Marais, Saint Germain des Pres, Montmartre,Pere Lachaise, The Ile de la Cite and the Ile Saint Louis. There are various "special" tours which might include almost anything- Malmaison, the Cathedral of Saint Denis, and even the Sorbonne, and the Academie Francaise (these last two are rather rare and occur only when they're open to the public). The guides are British or American. The tours I've taken have all been very interesting (the tours last about two hours, by the way) and informative, and the guides can be quite witty. They're perfectly willing to answer any questions you have. On the tours I've been on, the average group size is between 13 and 17 people, although I understand on one May 1 (a holiday in France), there were 57 people in the tour! Some of the other tours include the Latin Quarter, and Hemingway's Paris, and a couple of tours devoted to the French Revolution. Even if you've been to Paris before, they're worthwhile. There's even a Chocolate Tour, which I haven't been on. It's a bit more expensive but includes chocolate tastings!
p.s. I tried giving their French phone number, but when I tried to add this, you told me it was "invalid". read more