Almost every country with tramways/railways had some or all of that trackage as narrow gauge lines. These narrow gauge railways continue to exist in many parts of the world, either as operational railways or as heritage operations (tourist railways, museums, etc.).

Queensland Government Railways/Queensland Rail
QR operates passenger and freight services over 10,000 km of 1067mm (3' 6") gauge rail lines in the state of Queensland. Built as a series of narrow gauge resource lines to develop agricultural areas of the state, QR now depends upon coal traffic for much of its revenue.
Queensland's Shire Railways
Shire railways were owned and operated by local Shires, often either as an adjunct to the state rail system (QGR, now QR) or in the hope of state rail expansion into their area.
Sugar Cane Tram/Railways
These mainly 610mm gauge industrial railways have their own web site. While the CaneSIG site is oriented towards modellers it has a wealth of information for the railfan as well. [Direct links: Hawaii (3' 1947-8), Philippines]
Coal Creek Bush Tramway, Victoria
A 610mm bush tramway operating in a Gippsland heritage village setting.
Ida Bay Railway
Recently re-opened, this 6.8 km 610mm (2') gauge tourist railway in SE Tasmania started life carrying limestone.
Redwater Creek Steam and Heritage Society
With more than 1 km of track at Sheffield in Tasmania, this 2' gauge railway operates steam train rides using a Krauss 0-4-0.
Canadian Narrow Gauge
The Newfoundland Railway is perhaps Canada's best known narrow gauge railway but there were several others, including in the Pacific Northwest logging industry.
English and European Narrow Gauge
The well-known Welsh narrow gauge lines hauled slate from the mines to wharf but it is possible to find narrow gauge lines in almost every country and industry.
Gwalior Light Railway, India
This 300km+ 610mm railway started life as a Maharaja's 'toy'.
USA Narrow Gauge
Several 610mm (2') narrow gauge railways existed in Maine, but the best known are the 915mm (3') gauge mountain lines in Colorado (mining) and Pacific states (logging).
Timber/Logging Railways
Timber/Logging railways aren't always narrow gauge, but even the standard gauge ones generally operated much the same as their narrow gauge cousins.