Airspy SDR produce various SDR radios and their own software package 'SDR#' Their software also supports RTL-SDR radios
SDRplay have a line of excellent line SDR receivers that come with a great software package for their radios call SDRuno.
RTl-SDR.com is a blog style website with an extremely comprehensive catalogue of everything SDR related from software, hardware and everything in between.
NANA API is a c++ cross platform library containing many great helpers including graphic and math libraries. It is also used by the SDRplay plugin API.
This is a link to my public projects on GitHub. You can also find my free C# wrapper and library for the Contour Shuttle line of devices.