These are tools for users who own a physical software defined radio. For information on getting an SDR see Popular SDRs.
An open source SDR tool with a really nice UI. My personal favorite in this category.
Pronounced "SDR Sharp" (Think C#), this is a proprietary software from AirSpy. This one is very popular in the SDR community. It has a lot of features that other open source tools don't.
Another proprietary and less popular SDR tool but it works well so it's going on the list.
A popular open source SDR tool build with GNURadio.
A freeware SDR tool for Windows that has a pretty nice UI and a similar feature set to CubicSDR.
An open source tool that supports Rx as well as Tx. I've personally never got it running.
GNU Radio is a framework that enables users to design, simulate, and deploy highly capable real-world radio systems. It is a highly modular, "flowgraph"-oriented framework that comes with a comprehensive library of processing blocks that can be readily combined to make complex signal processing applications.
A an open source DSP framework that includes a giant list of SDR software for windows. Almost everything on this list (and more) is included for you here. If you are getting into SDR on Windows, you need this one.
The Universal Radio Hacker (URH) is a complete suite for wireless protocol investigation with native support for many common Software Defined Radios. URH allows easy demodulation of signals combined with an automatic detection of modulation parameters making it a breeze to identify the bits and bytes that fly over the air.
A tool for enabling remote access on SDRs.