You should always keep a list of useful telephone numbers handy.  Here are some crucial ones:

Police
999 (toll-free)

Emergencies/Ambulance/Fire Brigade
995 (toll-free)

Student Services Hotline
(65) 6831 3764
(operating hours 9am - 7pm, Mon to Fri)

Ministry of Education Hotline
(65) 6872 2220
(operating hours 8am - 6pm, Mon to Fri and 8am - 1pm, Sat)

Immigration and Checkpoints Authority (ICA) Hotline
(65) 6391 6100
(24-hour automated hotline for information on their services and procedures. Customer service officers are available from 8am - 5pm, Mon to Fri and 8am - 1pm, Sat)

STB Touristline
Tel: 1800 736 2000 (toll-free in Singapore only)
(operating hours 8.30am - 9.30pm, Mon to Fri ; 8.30am - 10pm, Sat ; and
11am - 10pm, Sun)

CitySearch (operator-assisted Yellow Pages)
Tel: (65) 1900 777 7777

International Calls
Tel: 104

Time of day
Tel: 1711

Trunk Calls to Malaysia
Dial 109 for operator assistance or 02 for direct dial.

Toll-free lines
Dial 1800 followed by the toll-free line number (in Singapore only).

Weather
Tel: (65) 6542 7788

Samaritans of Singapore (SOS)
1800-221-4444
(24hours Emergency Counselling)