There are quite a few, thought after the 4th game the others looked crap. Mainly because the 5th was done by another Konami team and the "Arcade" was a first person shooting game. The 6th looks like it's going back to where it belongs though.
There are:
1) Silent Hill
Conceived by the game designer Keiichiro Toyama, the first Silent Hill was released in 1999 for the Sony PlayStation. The plot of the game centered around Harry Mason arriving in Silent Hill and his subsequent attempts at finding his lost daughter, Cheryl. Similar plot with the movie.
2) Silent Hill 2
The sequel, Silent Hill 2, was released in 2001 for the Sony PlayStation 2, Microsoft Xbox and PC. The extended versions for XBOX and PC are sometimes known as Silent Hill 2: Restless Dreams and feature an extra subplot scenario, as do the "Greatest Hits"/"Platinum" budget re-releases for PS2. The plot centered around James Sunderland, who had received a letter from his deceased wife that she is waiting for him in their "special place", which leads him to Silent Hill.
3) Silent Hill 3
The second full sequel, Silent Hill 3, was released in 2003 for the Sony PlayStation 2 as well as for PC. Highly tethered to the events of the first game, the plot surrounds a teenager named Heather as she uncovers the mysteries that surround her past - including her own link to the haunted town. The story follows the plot of the 1st game.
4) Silent Hill 4: The Room
Silent Hill 4: The Room was released in 2004 for the Sony PlayStation 2, Microsoft Xbox and PC. Partly related to Silent Hill 2 and briefly set near Silent Hill, the plot follows Henry Townshend who finds himself mysteriously locked in his own apartment until a hole appears in his bathroom wall. The game features characters that are mentioned in previous titles.
5) Silent Hill Origins
An upcoming prequel first announced at E3 2006 for the PlayStation Portable.The game features a mentally disturbed trucker identified as Travis O'Grady who becomes trapped in Silent Hill and encounters characters from the first game during his attempts to get out. As the protagonist searches for a way to escape the horrors of the town, he eventually uncovers details regarding the creation of the darkness that plagues the once peaceful Silent Hill.This is one of the few Silent Hill titles that is not being created by Team Silent, but instead by Climax Studios.
6) Silent Hill: Arcade
At the 2007 Japan Arcade Operator's Union trade show, Konami revealed a forthcoming light gun arcade shooter game based on the Silent Hill series. The game deals with two characters, Eric and Tina, who have entered the town of Silent Hill and must battle enemies which include Nurses and Pyramid Head from Silent Hill 2. It is said that the two-player arcade will play much like House of the Dead.
7) The sixth installment was officially confirmed on July 11th, 2007, during E3 2007 for the Sony PlayStation 3 and Microsoft Xbox 360, but still hasn't been released. The game tells the story of Alex Shepherd, a soldier who has returned from a war overseas. Shepherd discovers upon his arrival that his father has gone missing and his mother catatonic. Joshua, his brother, is the only one who can provide solace, but he soon vanishes. The game will chronicle Shepherd's search to find his missing brother. Much like Silent Hill: Origins, Team Silent will not be developing the project, but instead by The Collective from Foundation 9 Entertainment. But for some reason it looks fairly good. We'll have to wait and see.