A subspace receiver is a piece of Tok'ra technology that relays information from tagged positions and Tok'ra operatives via subspace.
The technology was stolen by Jacob Carter/Selmak from the Tok'ra and given to the Tau'ri to track Ba'al's fleet and watch the progress of his war with the Replicators. The technology was seen again when Samantha Carter rigged it to emulate the Long-range communication stones built by the Ancients and allow Vala to communicate with Earth via Daniel Jackson. It is unknown if this was the same subspace receiver that Jacob Carter/Selmak gave them or if it was another officially given to them by the Tok'ra. (SG1: "Reckoning, Part 1", "Crusade")