Here are some of my favorite reources for making life in Paris go a little more smoothly.
1. www.pagesjaunes.fr Obvious for a number of reasons, including the predictably useful pagesblanches tab. Extra astuce: if you can't remember the name of the place, just plug in the street name and browse through the listings. Also excellent for looking at aerial views of where you and all of your pals live. And webcams of various parts of Paris. Great map feature.
2. www.ratp.fr I can't live without this. Best ways to get across Paris by public transportation. Plug in your destination and preferences (mode, fewest connections, fastest, least foot travel, etc.). Voila!
3. www.mappy.fr This of course is the French equivalent to MapQuest, but I find it much more user-friendly. Great for those of us who like to get around on foot, but equally good for getting yourself out of town by car.
This site from the Prefecture de Police of Paris will tell you which of the above transportation routes are disrupted this week by local manifs, defiles, visites d'etat, and so on.
This tells where all the outdoor markets are, by arrondissement. Basically, getting to know the www.paris.fr website is the most bountiful resource of all.