The definition of Web 2.0 varies depending on who you ask. But it seems that everybody agrees that tagging, social networking, RSS, blogging, bittorrent and context sensitive ads are part of Web 2.0.

To summarize I suppose that you could say that Web 2.0 are the things that can help make the web a social place, where you can actually interact with others.
This diagram attempts to explain Web2.0
That perspective should be compared to how the web was percieved during its first five years of existance (in the public sphere.)
During that period people came online to research, shop and perpaps participate in a forum.

Technologies and attitudes over the last five years or so has changed all that though. Active participation of users is now the most desirable state of things on most websites. Thats is what Web 2.0 is all about. Individuals. organisations and companies no longer wants to simply tell you things. They want you to get involved as well!

Below is a list of some of the most famous Web 2.0 sites today. I have left out some of the obvious ones that I expect you already know about; Wikipedia, Technorati, YouTube and MySpace….

Site	Description
Wink	Search all the big social networking sites such as MySpace, Facebook, Bebo, Xanga, LinkedIn (and many others) in one go. Much more practical than trawling through these sites one by one looking for your friends (or for a blog in a specific location or on a topic that interests you.)
Linked In	Social Networking for professionals. Recruitment consultants love it. It’s cheesy like you wouldn’t believe, but it actually is pretty good for getting head-hunted. I get a few quite relevant phonecalls every week, just from having an entry there.
Last FM	A community music site that offers you good quality streaming radio based on music that is recommended to you. The recommendations are based on what others with similar taste to you have liked previously. The site offers plenty of opportunities to make online friends and participate in groups based on music taste.
Pandora	Music selections streamed based on a computer’s selection. All music in Pandora has been analysed and categorised by the Gnome project. As you give the songs played the thumb-up or thumb-down, the Pandora software quickly learns what type of rythms you like, as well as what type of genres, lyrics, instruments and tonal scales please you. Within half an hour, Pandora’s got your music taste all sussed out and is able to suggest new artists and songs that you are guaranteed to like. Well done Gnome - I love Pandora!
Flickr	Photosharing. Upload your photos, tag them and share them with the world. Anybody searching for ‘Paris’ will be able to find your snapshots of a certain tower… The comment functionality allows people to let you know what they think - and they do, usually in an encouraging way. Flickr appears to be big among professional photographers and aspiring professionals. I currently don’t use it.
BTJunkie	Somebody researched all the bittorrent sites and found that this site is the most comprehensive. Personally I tend to use Torrentspy, Demonoid or The Pirate Bay though. In order to use the notorious bittorrent technology to download films, music and software you have to install a bittorrent client such as uTorrent. Once it is installed, you simply visit a BT site, search for whatever it is that you want, click on the link and you are in business. In order to get a good performance, you need to do some tweaking though.
9Rules	A sort of ‘elite’ blog network. A blog that is considered for membership gets thouroughly scrutinized by the 9Rules team. The site can only join if it measures up to some farirly high standards. I would like to join but they currently don’t even accept new applications. There is no guaranteed that my site even measures up!
Del.icio.us	I didn’t use this “social” bookmarking service for ages because I thought the name was silly. But it is actually pretty good. If you can manage to find somebody with who has a great set of bookmarks, you can learn a lot on a rainy day… I haven’t used it to it’s full extent but lots of people swear by it. Timewaster warning!
Stumble Upon	Stumble will bring you to random sites, based on your interests. Tell this service what type of sites you would like to view, then sit back and click the ‘Stumble’ button in your browser’s toolbar as you surf sites that have been recommended by members. If you come across something that you don’t like, just give it a thumbs-down. Again, Timewaster warning!
Second Life	If this life isn’t enough for you, you can have a Second Life online. Why would you want that? I have NO idea, but lots of people do… The second life has everything that the first life has; love problems, fashion, private property, airports and a currency. Do I need to say Timewaster warning?
Mybloglog	This community site is about finding out more about the people who read your blog (see the ‘visitors’ section in the right-hand column. Alternatively, it lets you find out about those who read blogs that you like, such as here on this site. By clicking on their picture, you can find out all about what their online likes and dislikes are, if they have their own blog as well as links to their pages on lots of other web2.0 sites. It is not an intrusion of their privacy, since they get to decide what information is public and what is not. The service only works with people who signed up. I like the feature that lets me add a module that shows the last readers.
Pageflakes	Your personalised start or newspage. Lets you keep up with the activity on your favourite blogs and news sites without surfing around like a madman. Simply add the RSS feed from your favourite pages to Pageflakes and you can spot any updates to the sites from there. A particularly nice touch is that you can create new tab pages for different topics. For example you may have one tab for blogs, one each for specific topics that interest you, and one for news weather and local events. Also check out Netvibes which does the same thing as Pageflakes. Note: To use this site successfully you should understand the basics of RSS. There is nothing difficult about it - just look it up on Wikipedia.

Hope you found the list useful! Please let me know if I missed anything that you think is important, and I’ll update the list.
It took me some time and effort to research this a couple of weeks back and I hope that it will save sombody else a bit of time to see a comprehensive list instead of finding scraps or information here and there.

And finally: The ten commandments of Web 2.0 at Lohad’s site.
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