There are 2 types of shared accommodation in SG and both are advertised using the same means, but you can usually work out which is which.
The first is more "boarding house" type accommodation - you have your own room (or even share a room) but its not very communal and quite often cooking is not allowed. There are also dormitories - these are generally easy to find and ignore by price ($200-$600 per month) which can have many more people to a room. Then you get the traditional western-style shared accommodation which is probably priced from $1k (common room) to $2k (master room) depending on the area. Just look through the ads and see what you like.
The other funny about shared accommodation in singapore is that agents are involved. Generally they're more involved with the boarding house type accommodation than the other. Be careful - agents have a bad reputation in Singapore (worse than used car salesmen!) and my experience is that I'll put the phone down on any that call me. If you use an agent, you will pay 1 month rental as commission (over and above deposit) to the agent so you need to factor that one in too.
The websites to look for are here:
http://www.gumtree.sg more dormitories-style, but do get the occasional other type
http://www...com.sg easy to identify subscribed users for easy contact
http://www.share-accommodation.sg cannot identify subscribed users, must be subscribed to contact, annoying selection criteria
http://www.craiglist.org more expat focused but not a large selection
http://classified.singaporeexpats.com more expat focused, but not a large selection