I have allways been fascinated by Ferrites and their uses specially in Transmission Line Transformers as Broadband transformers, an area that many hams are not familiar with or understand as to why they are superior devices.
Link to chart on Ferrite Type qualities.
Link to a chart with core sizes and properties of Ferrite cores.
Link to a chart with core sizes and properties for Binocular Ferrite Cores.
Below are two papers by Fair-rite on using ferrites;