Build Your Own ASP.NET Cache with WeakReferences

I very much like the ASP.NET Cache and the more generally usable MemoryCache collections: They let me build up a set of objects (typically drawn from a database) that I can use in my application. Of course, those objects take up room in memory, which is too bad -- especially if the computer is running short on memory. However, the nice thing about these two caching objects is that they discard items in the collection when memory gets tight.