The following are alternatives to streetdirectory.com . Blog has been updated as of Aug 15, 2015. Dead links as well as redundant materials has been removed. New links has also been put up.
Disclaimer: All info was compiled to the best of my knowledge as of last update. I cannot be held responsible for any errors. This blog is not affiliated to Streetdirectory.com in anyway.

Google Maps (Singapore):
http://maps.google.com.sg

Street view has now been fully implemented into Google maps.

Google has partnered with Singapore Land Transport Authority with the latter providing up to date traffic/transit information as well as public bus/MRT routes and schedules for implementation into Google Maps.

Very useful for the planning of driving route from starting point to destination. Just click on the "Get directions" link. Make use of the navigation menu on the left and the drag ruler to zoom in/out. There's even a option to avoid highway. You can form an alternative route by dragging "A" and "B" and the circles formed when pointed at the blue line. Right click on the circles to select "remove this destination" or "add a destination" option. Results even includes mileage. Directions for walking is now also included.

Double left click on the map to centralise and zoom in. Double right click to zoom out. Left click and hold to drag the map.

You can also right click on a particular location to display a menu with extra options. (options include "Directions from here" "Directions to here", "Centralise map here", "Zoom in/out")

Users of Firefox should press the ESC button on your keyboard after right clicking to reveal the menu.

Convert between satellite, map or terrain view. Satellite button has a show label function.
I can search for terms like for example "wisma atria" and it displays the exact location.

Good for the planning of driving route but map data can sometimes not be up to date. Able to search using keywords.


SLA Streetmap:
https://www.onemap.sg/
A street directory alternative affiliated with the Singapore government.

StreetDB:
http://www.streetdb.com/ 
Powered by Onemap and Google Maps.


Singaporeexpats:
http://maps.singaporeexpats.com/
Powered by Onemap and Google Maps. 


ArcGIS:
http://www.arcgis.com/home/webmap/viewer.html
Powered by Openstreetmap with data from ArcGIS. Option to change the base map.


Gothere:
http://gothere.sg/
Features of gothere.sg

1. Includes bus service/bus stop info as well as arrival time. Just click on the bus icon (some express bus services still missing).

2. Displays location of taxi stands in CBD. Look out for the orange icon.

3. Location and rates of car parks as indicated by a "P" icon.

4. Location of ERP gantries and rates has also been added as indicated by a "ERP" icon.

5. Driving directions now includes option to avoid erp gantries as well as the ability to calculate rates based on the time of a journey.

6. Search for directions by taxi, with a estimate of fare/surcharges.

7. Great directions search from starting point to destination. 


Here Maps Singapore:
https://wego.here.com/
Powered by Nokia.


Openstreetmap Singapore:
http://www.openstreetmap.org/#map=12/1.3328/103.8307&layers=T
Open source street map of Singapore.


URA Maps:
https://www.ura.gov.sg/maps/
Option to select different base maps.