Base types, Collections, Diagnostics, IO, RegEx…
Krzysztof Cwalina and Kathy Kam are two BCL Program Managers that also have their own blogs.
Krzysztof recently wrote about extending System.Diagnostics tracing APIs in the April Issue of MSDN Magazine.
Kathy recently wrote an introduction to format strings in .NET, a topic that new .NET users often found confusing.
Check them out for other BCL related content.