Appearance: Venus #1 (August 1948).
Golden Age Appearances: Venus #1-19, Marvel Mystery Comics #91.
Modern Appearances: What If v1 #9, Sub-Mariner #57, Champions #1-3, Avengers #283-285, Marvel Valentine Special #1, Marvel Super-Heroes v3 #9, Avengers Forever #3-6, Marvel: The Lost Generation #5.
Years Active: Dawn of time-present.
The Roman goddess of love, Venus lived for centuries on Mount Olympus with the other Roman gods and goddesses. After World War Two, she realised that the world was in need of her powers of love, and so traveled to Earth to make the world a better place.
During the Golden Age her adventures were solo, albeit interesting, but when she was brought back in the modern era her adventures after World War Two were added to. After various solo adventures Venus was contacted by FBI agent Jimmy Woo and recruited to fight alongside Gorilla-Man, Marvel Boy (III), the Human Robot, and 3-D Man as the Avengers against the Yellow Claw.
Then came the Retcon Hammer, I mean, Avengers Forever, which erased this adventure. In current Marvel continuity Venus and the rest of those indviduals were still active in the 1950s (see the Human Robot entry for why this is probably not such a great thing), but they never formed together as a team.
Ronald Byrd, who knows everything, pointed out that the Official Handbook of the Marvel Universe does indeed acknowledge Venus' first appearance, and that the Valentine Special which Venus appeared in exists in continuity, so her adventures during the 1950s are considered canon (at least until they are denied in a future sequel to Avengers Forever, I suppose).
Note: It was said, in the Golden Age stories, that Venus actually lived on Venus, presiding over Cleopatra and Salome, who were her handmaidens, and acting as the "Supreme One" over even Juno. We must assume, I think, that this has been retconned away, in much the same way that the issue in which the Human Torch (I) flew to Jupiter (or was it Saturn?) has been retconned away.