known slayers

From the shows
Beginning of time
The Primal Slayer (seen in "Restless" and "Intervention")
China, 1900
Chinese slayer (seen in "Fool For Love")
1860s
Lucy Hanover
Chicago, 1920s
Unnamed slayer
Korea, 1930s
Unnamed slayer
New York, 1977
Nikki (seen in "Fool For Love")

From the Tales Of The Slayers graphic novel
England, Middle Ages
"One maiden, dressed in white" (seen in "Righteous")
Paris, French Revolution
Claudine (seen in "The Innocent")
England, Regency Era
Elizabeth Weston (seen in "Presumption")
California, mid-1800s
Naayéé´neizgháni (seen in "The Glittering World")
Germany, 1938
Katrina Sonnenblume (seen in "Sonnenblume")

From the Tales Of The Slayers, Vol. 1 anthology
Greece, 490 BCE
Thessily Thessilonikki (seen in "A Good Run")
Roanoke, 1590 CE
White Doe (seen in "The White Doe")
Hungary, 1609 CE
Ildikó Gellért (seen in "die Blutgrafin")
France, 1789 CE
Marie-Christine Du Lac (seen in "Unholy Madness")
Kentucky, 1886
Mollie Prater (seen in "Mornglom Dreaming")
Germany, 1923
Britta Kessler (seen in "Silent Screams")
Florida, 1956
Asha Sayre (seen in "And White Splits The Night")

From various other novels
Greece, 1527
Cassia Marsilka (seen in Tempted Champions)
Denmark, 1940
Sophie Carstensen (seen in Pretty Maids All In A Row)
Barcelona, 1940
Isabel Cortés (seen in Pretty Maids All In A Row)
New Orleans, 1940
Eleanor Boudreau (seen in Pretty Maids All In A Row)
1990s
India (seen in The Book Of Four)

Please note that the Tales Of The Slayers anthology and the other novels were not written by the general body of BtVS/Ats staff writers and that the show may change these details at any time.
Thanks to Christina and Gabe for some of these details.