1. General: Downspouts: “Closed Faced vs Open Face Downspouts in Colder Climate Regions” is short but informative. From the Metal Era website.
2. General: Gutters: The Wikipedia article here provides an accurate overview.
4. Maintenance: "General Inspection And Maintenance Of Gutters And Downspouts". Available on the website of the US General Services Administration.
6. Technical: “Demystifying Gutter and Downspout Sizing” is a useful little article from the Architectural Graphic Standards website. Helps you understand the equations you need to use and how to use them.
8. Technical: "Appendix B: Rates of Rainfall for Various Cities" on the ICC website provides the rainfall data for major US cities required for the sizing calculations mandated in the building codes. If your town is not listed, visit NOAA's Precipitation Frequency Data Server (PFDS) to look up the 100-year one hour rainfall data for your area.
9. Technical: "Gutters and Downspouts" can be seen at the website of the Copper Development Association, and includes an explanation of the process of determining the proper dimensions of a hypothetical gutter. Be aware that the data in the Expansion Joint section is specific to copper gutters.
10. Manufacturers: Gutter manufacturers can be found here.