A lot of great stuff can be found at residential areas in Singapore.
Large pieces of trash (furniture, unwanted consumer electronics) are often left at the front of houses where it usually gets picked up by the professional karang guni (trash recycler) people. They normally do their collections on the weekends, cruising around the neighbourhood with a pickup truck and sounding a horn.
HDB flats (government housing)
The majority of Singaporeans live in blocks of flats. There are centralised trash collection centres for each group of flats. Good trash can sometimes be found there, but given that everything ends up in the collection centre, dumpster diving is often a stinky, unpleasant experience.
The Thieves Market at Jalan Besar is one of the largest flea markets in the country.