Although plastic figures are commonplace now, I believe Ral Partha was the first company to produce plastic fantasy game figures.
(Of course, there have always been plenty of plastic toys, but I think these were the first plastic figures made specifically for RPGs.)
There were 4 figures, elementals from the AD&D line, that were made in a translucent plastic for Fire, Water, and Air. The Earth elemental was made in an opaque plastic.
The plastic figures were issued as 11-527: Elementals of Fire and Earth, and 11-528: Elementals of Air and Water. They are identical to the metal elementals issued as 11-426 (Fire & Earth) and 11-427 (Air & Water).
I bought the plastic figures at the Ral Partha booth at Gen Con. They were just loose pieces on the table and I thought the transparent plastic made a really good elemental figure, especially for water. I guess there weren't a lot of people who agreed with me, or maybe manufacturing the plastic figures was too difficult or too costly, but Ral Partha never produced any other plastic figures.
**Update: It turns out I was wrong about Ral Partha not making any other plastic figures, there are actually two more. (Both of them from the AD&D line.)
The first is the Banshegh, from the metal and plastic combo set of Justina Heulough and the Banshegh (11-753). Justina is a sort of "Dr Jekyll & Mr Hyde" character in the Birthright campaign setting. She is the ruler of Pashacht by day and an evil spirit by night.
The other is a ghost version of Syluné Silverhand (11-086), eldest of the Seven Sisters from the Forgotten Realms setting.
You can see the figures on Alan Stewart's web page, and at the Lost Minis Wiki: Justina, Banshegh, Sylune.