From time to time I end-up finding really old hidden gems and a few days ago I found another one.
IList System.Collections.ArrayList.ReadOnly(IList list)
This amazing method is available since the beginning (.NET 1.0).
I always complain about the small support for ReadOnly lists and collections and I have no clue why I miss this one.
For those of you that have to maintain and extend legacy applications prior to ASP.NET 2.0 SP2 this could be a very useful finding.