A lot of people have blogged about System.NET tips and tricks. Esp from Microsoft, prior System.Net team members (Jon Cole, Durgaprasad Gorti, Malar Chinnusamy) have authored blog posts on how to do stuff with System.Net.
Non microsoft folks have also stepped up to the plate. For eg, Stephen Cleary has written a very good concise blog on System.Net HOWTOs and FAQs.
In this post, I aim to categorize all of them and link them in one place, so that it will be easy to find for people who need information.
Stephen Cleary's .NET TCP/IP Sockets FAQ
How to use a Network Monitor to debug your System.Net application
Creating a tracelog for your application
Proxy Configuration Best Practices in System.Net
Article on preserving message boundnaries
Another excellent sample on message framing by Stephen Cleary
How to use CookieCollection/CookieContainer
HTTP Expect-100 Continue Caveats
The following blog post explains the consequences of having Expect100Continue=true for some servers.
Socket duplication is a new feature supported by the 2.0 version of the .NET framework. The following articles describe that in more detail, along with code samples.
Socket Duplication - Part 1
Socket Duplication - Part 2
Configuring an IPv4 server to be able to receive connections from IPv6 clients
http://www.systemnetmail.com/default.aspx (FAQ for system.net.mail namespace and classes)
Sending mail to Gmail a/c with SMTPClient
Socket Applications: Traceroute
HOWTO send an object across the network, between a Java app and a .NET app, and back.