Slytherin House

Head: Professor Severus Snape until spring of 1997, then Horace Slughorn.
Founder: Salazar Slytherin.
Blazon: vert, a serpent argent (that is, a silver serpent on a green field, hence the Slytherin colours of green and silver) (PA15, GF15). The serpent device signifies the founder's gift of being a Parselmouth (CS11).
Ghost: The Bloody Baron (PS7).
Location: The Slytherin common room is a low-ceilinged, dungeon-like room with greenish lamps and chairs, with skulls all around. It is located under the lake (DH23) [1].
Sorting Hat description: (PS7)

Or perhaps in Slytherin
You'll make your real friends
Those cunning folk use any means
To achieve their ends.
Slytherin Quidditch team
green robes
players chosen more for size than finesse or skill, it seems
all fly on Nimbus 2001s
the 1991-92 team included no girls

Known Slytherins, past and present

For each known Slytherin below, his or her name is given (with a link to the appropriate entry in Which Wizard), together with the years he or she was a student in Slytherin. See the individual students' entries for more detailed information (such as the references for how we know that they're Slytherins and what years they were in).

(unknown, but roughly contemporary with the Founders)

The Bloody Baron
(years unknown, but possibly nineteenth century)

Slughorn, Horace
1858-1866

Black, Phineas Nigellus
(1936 - 1943) (or is it 1938?)

Avery (Riddle's contemporary)
Riddle, Tom Marvolo became Lord Voldemort
(c. 1970s)
Avery (Snape's contemporary)
Black, Bellatrix (c. 1970s)
Black, Narcissa (c. 1965 - 1972)
Black, Regulus (c. 1972 - 1979)
Lestrange, Rodolphus (c. 1970s)
Lestrange, Rabastan (c. 1970s)
Malfoy, Lucius (1965 - 1972)
Rosier, Evan (c. 1970s)
Snape, Severus (1971 - 1978)
Wilkes (c. 1970s)
(1990s)
Bletchley, Miles
Higgs, Terence
At least a year ahead of Harry, since he played Seeker in Harry's first year.
Urquhart (HBP14)
No more than a year older than Harry, since he was on the team in Harry's sixth year. May be younger than Harry.
Vaisey (HBP14)
No more than a year older than Harry, since he was on the team in Harry's sixth year. May be younger than Harry.
1986 - 1994
Flint, Marcus
Repeated a year without being kicked off the team, so actually left Hogwarts at the end of Harry's third year rather than his second. This error was fixed in later editions of the books.
1988 - 1995
Bole
Derrick
1989 - 1996
Montague
Pucey, Adrian
Warrington, C. (c.1989 - 1996)
1991 - 1998
Bulstrode, Millicent
Crabbe, Vincent
Davis, Tracey (witch) (HPM)
Goyle, Gregory
Greengrass, Daphne (HPM, OP31)
Malfoy, Draco
Nott, Theodore
Parkinson, Pansy
Zabini, Blaise
1992 - 1999
Harper (male)
1993 - 1994

Astoria Greengrass (first name uncertain, could be Asteria, sister to Daphne) (JKR)
1994 - 2001
Baddock, Malcolm
Pritchard, Graham