I was helping a mentoring client translate some sample code from VB to C# - he has written a lovely library and a customer wanted to see C# code that called it. The original samples had been written by someone who wasn't in the room, so I could have a bit of a vent. They had made heavy use of the Microsoft.VisualBasic namespace (nothing wrong with that) so as to access familiar (to a VB6 programmer) string manipulation methods. The problem was, these methods were being used to do things like finding the extension given a file name, or extracting a file name from a long path. This work should never be done with Left, Right, and Mid - and using Substring doesn't make it better. There are all kinds of methods in System.IO that manipulate paths and file names in a more robust and localized way than you can possibly do by hand. For my client's code, I ripped out a whole pile of hand written code (including a loop that looked character by character at a string testing to see if each was '\' or not) and replaced it with calls to library methods.
Phil Haack lists a bunch of these (and his commenters even more) in a very useful post. Developers who know what's in System.IO and the other meat-and-potatoes parts of the .NET Framework will write code more quickly, and that code will be better. Never assume you're the first person who wanted to parse a file path, check if a string is a good date, or any of a thousand other general tasks. Trust me, what you want is in there ... look for it.