What the heck is "Web 2.0" anyway?
Here is a brief introduction to some of the more famous Web 2.0 services:

http://digg.com/
Digg is a place for people to discover and share content from anywhere on the web. From the biggest online destinations to the most obscure blog, Digg surfaces the best stuff as voted on by our users. You won’t find editors at Digg — we’re here to provide a place where people can collectively determine the value of content and we’re changing the way people consume information online.

http://www.linkedin.com/
LinkedIn is an online network of more than 25 million experienced professionals from around the world, representing 150 industries. When you join, you create a profile that summarizes your professional accomplishments. Your profile helps you find and be found by former colleagues, clients, and partners. You can add more connections by inviting trusted contacts to join LinkedIn and connect to you.
Your network consists of your connections, your connections’ connections, and the people they know, linking you to thousands of qualified professionals.

http://www.google.com/talk/
The Google Talk gadget is a web-based application with some fun features for communicating in real time.
- Instant messaging — Chat with all of your Google Talk and Gmail contacts in real time.
- No download required — Start chatting immediately from any computer.
- Group chat — Invite multiple people to chat with you in a group.
- Media previews — Cut and paste video and slideshow URLs from YouTube, Google Video, Picasa Web Albums and Flickr into your chats and view them in your chat window.
- Add to iGoogle — You can add the Google Talk gadget to your iGoogle homepage to put chat alongside other useful and entertaining gadgets.
- Add to your own webpage — Just cut and paste this code into your own webpage or blog to embed the Google Talk gadget.

http://www.google.com/reader
Keep track of your favorite websites. Stay up to date.
Google Reader constantly checks your favorite news sites and blogs for new content. Whether a site updates daily or monthly, you can be sure that you won't miss a thing. Simplify your reading experience. Google Reader shows you all of your favorite sites in one convenient place. It's like a personalized inbox for the entire web. Discover new content. Millions of sites publish feeds with their latest updates, and our integrated feed search makes it easy to find new content that interests you.

http://www.reddit.com/
Reddit is a source for what's new and popular on the web -- personalized for you. Your votes train a filter, so let reddit know what you liked and disliked, because you'll begin to be recommended links filtered to your tastes. All of the content on reddit is submitted and voted on by users like you.

http://www.slideshare.net/
SlideShare is the world's largest community for sharing presentations.
- Individuals & organizations upload presentations to share their ideas, connect with others, and generate leads for their businesses.
- Anyone can find presentations on topics that interest them. They can tag, download, or embed presentations into their own blogs & websites.
SlideShare is the best way to get your slides out there on the web, so your ideas can be found and shared by a wide audience. Do you want to get the word out about your product or service? Do you want your slides to reach people who could not make it to your talk? Are you a teacher looking to share your lesson plans? It only takes a moment - start uploading now, and let your slides do the talking!

http://www.stumbleupon.com/
StumbleUpon helps you discover and share great websites. As you click Stumble!, we deliver high-quality pages matched to your personal preferences. These pages have been explicitly recommended by your friends or one of 5,916,656 other websurfers with interests similar to you. Rating these sites you like automatically shares them with like-minded people – and helps you discover great sites your friends recommend. How Does it Work? StumbleUpon uses ratings to form collaborative opinions on website quality. When you stumble, you will only see pages which friends and like-minded stumblers have recommended. This helps you discover great content you probably wouldn't find using a search engine.

http://twitter.com/
Twitter is a service for friends, family, and co–workers to communicate and stay connected through the exchange of quick, frequent answers to one simple question: What are you doing? Tell us what you're doing in 140 characters or less! Send your thoughts, observations, and goings-on in your day. Whether you're "eating an apple" or "looking foward to the weekend" or "Heading out of town" it's twitter-worthy.

http://friendfeed.com/
FriendFeed enables you to keep up-to-date on the web pages, photos, videos and music that your friends and family are sharing. It offers a unique way to discover and discuss information among friends. Sign up for FriendFeed, invite some friends, and get a customized feed made up of the content that your friends shared — from news articles to family photos to interesting links and videos. FriendFeed automatically imports shared stuff from sites across the web, so if your friend favorites a video on YouTube, you get a link and a thumbnail of the video in your feed. And if your friend likes a news story on Digg, you get a link in your feed. FriendFeed makes all the sites you already use a little more social. It’s also fast and easy to start discussions around shared items. On FriendFeed, you and your friends contribute to a shared stream of information — information that you care about, because it's from the people that you care about.
You don't need to install anything to use FriendFeed. But if you already use Facebook, you can add our Facebook application to connect your Facebook profile to all the other products you use around the web. You can also view your FriendFeed in your iGoogle homepage or read it in a feed reader. If you make your FriendFeed publicly visible (kind of like a blog that writes itself), you can embed your feed in your home page or blog.

http://www.new.facebook.com/
Facebook gives people the power to share and makes the world more open and connected.
Millions of people use Facebook everyday to keep up with friends, upload an unlimited number of photos, share links and videos, and learn more about the people they meet.

http://www.blogger.com/home
We created Blogger to give you an easy way to share your thoughts — about current events, what's going on in your life, or anything else you'd care to discuss — with the world. We've developed a host of features to make blogging as simple and effective as possible.

http://flickr.com/
Flickr - almost certainly the best online photo management and sharing application in the world - has two main goals:
- We want to help people make their content available to the people who matter to them.
- We want to enable new ways of organizing photos and video.
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Are there any other web 2.0 applications that we did not mention that use and recommend? Please let us know about them.