The Agena was a series of upper stages used with Thor, Atlas, and even Titan boosters (although I am unaware of any current museum display consisting of a Titan with an Agena).

The Agena came in three variants, the Agena A, Agena B, and Agena D (the Agena C, a proposed up-rating of the Agena B, was never built). The A and B variants were used to launch a number of "Discoverer" satellites, which were a cover story for the Corona reconnaissance satellite program.

Agena A configuration
Click image for a 6044x2284 pixel version of this image in a new window.
Taken from p. 14/15 (p. 17/18 in the PDF) of the Agena
Extraction, reconstruction, and clean-up by heroicrelics.org.

In addition to the Discoverer/Corona program, the Agena B also served as an upper stage for the Atlas booster to launch several Ranger probes to the moon and Mariner 2 to Venus.

An improvement in the Agena B was that its engine, which used unsymmetrical dimethyl hydrazine (UDMH) as the fuel and inhibited red fuming nitric acid (IRFNA) as the oxidizer, could be restarted in orbit.

Agena B schematic
Click image for a 1354x932 pixel version of this image in a new window.
Taken from p. 237 (p. 248 in the PDF) of the Technology and the Air Force: A Retrospective Assessment
Extraction and clean-up by heroicrelics.org.

In addition to being used as a satellite launcher, the Agena D was used in NASA's Gemini program as a docking target. Gemini 8 became the first spacecraft to dock with another spacecraft.