Starting with the obvious, here is the official Tour website http://www.letour.fr/ though it's not extremely user friendly. You can find the neutral take on the race here. Also if you happen to be working during the Tour de France but you are like me and spend considerable amounts of time sneakily keeping up to date with goings on via the office Internet (I won't tell if you don't!) then there is a very useful animated ticker with up the minute, as it happens race commentary.
Next on the essential list is the Inner Ring Tour de France guide http://inrng.com/tour/ which is always one of (if not the) best anywhere on the net. Constantly insightful and always up to date Inner Ring could easily provide a one stop shop to the Tour regardless of the preview. Visit every day to see the Inner Rings take on the latest movers and shakers.
With the Tour comes 'silly season' - the rumour mill works overtime with constant talk of rider transfers throughout the Tour as riders, team management and sponsors all come together in one place. Along with this there's bound to be a scandal or two, more often than not doping related. More often than not http://www.velonation.com/ is first on the scene for all gossip of this type.
If you get bored on a rest day and fancy brushing up on your knowledge
http://en.wikipedia.org/wiki/Tour_de_France will provide all the Tour history you will need. handy for the post race pub quiz also.
A comical take on the Tour can usually be found over at http://www.atwistedspoke.com/ especially where Frank and Andy Schleck are concerned. Unfortunately Andy isn't riding this year, so it look's like Frank may take all the stick!
If It's a rider startlist, highlights, live video stream or some juicy photographs that you need, head on over to http://www.steephill.tv/ and click 'Tour de France' for the most op to date offerings. http://www.cyclingfans.com/ also has a long list of live streaming to appreciate.
If you have missed a stage and need a brief summary then http://www.cyclingnews.com/tour-de-france is usually the first major news site to get the full results and a write-up online.
If after watching you cannot contain your excitement and feel the need to share your own views and opinions on the days events then a decent cycling forum is the place to be. I've found that the 'Road - Pro-Race' section here http://www.bikeradar.com/forums/index.php?c=4 is one of the most informed and more importantly the most friendly on the web. Whether a newbie or a know-it-all there will be plenty of talking points over here. Some users are a little closer to the action than others and often provide some inside information than isn't available elsewhere.
If you like to get bogged down in the numbers then Cycling Quotient
http://www.cqranking.com/men/asp/gen/start.asp will tell you whose ridden how many miles and for whom this season. A good indicator of form and preparation.
If foreign language is your thing then unlike Britain, the press over in France, Belgium and Italy take cycling far more seriously. If there's breaking news and differing opinion it's like to be found here first. if language isn't your thing then Google translate is quite an efficient tool http://translate.google.com/. For French http://www.lequipe.fr/, Belgian http://www.sporza.be/cm/sporza, and Italian http://www.gazzetta.it/ its all here.
If you are a regular on Twitter then there is some 'Must-Follows' during the Tour and beyond. Besides the usual cycling news twitter accounts, be sure to add @inrng @Festinagirl @Vaughters @mrendell @UCI_Overlord @NYVelocity and @nedboulting for plenty of opinion.
I could make this endless but unfortunately I don't have the time. The above should provide a pretty rounded view of the Tour de France whether a seasoned veteran or a complete newcomer. Remember I'm on twitter too at @InTheGC and the blog will be providing daily coverage of the tour so be sure to check back as often as you like. Enjoy the links and enjoy the next 3 weeks!