After having good experiences with the various websites for comparing flight prices (Expedia, Orbitz, etc), I was wondering whether there's anything similar for tours that compares times, prices, itineraries, etc? I have a handful of tour operator websites that I check but I can obviously only check the ones I know about.
I found tour radar was quite useful for finding different tours from non main stream companies as well as all the regulars. You can choose tours that go through a specific country or ones based in just the country you select. There are also plenty of options to weed out those that aren't suitable, such as age, duration, price, trip type, group size, dates and operators.
Comparison website for tours are hard-to-find because of the unstructured nature of the industry. Flights, trains et al have fixed schedules which makes it easy for travel comparison websites to easily sort and compare things. Most online tour booking sites are listing websites as there's no standard way of representing the information, and more importantly the actual tour operators don't often themselves have comupterised management systems (I'm referring specifically about SE Asia).
For SE Asia, two major travel sites that will give you tour comparisons would be Wego and Air Asia Go. Wego does a somewhat good job of giving a few tools to sort and compare tours - at least what I have seen so far on tour sites; Air Asia Go is a more of static listings offered by Air Asia (you don't have to fly with them to buy tour packages). If you want to spend on mid to high range budget tours for SE Asia, start with these two places.
You will be better off booking on the ground in each country in SE Asia though. The prices mentioned on these sites are hugely inflated; basically, an online tour booking agent offering local tour operators a new medium for sales in exchange for a huge markup in prices which the online tour operators keep. If you book online, you may end up with the same tour operator as one you could book with directly, and yet end up paying up to 3-10x the amount.
I've found TourAmigo (http://www.touramigo.com/) to be the best tour comparison site for multi-day tours. They cover 10 different continents and all the small and major tour operators and allows you to contact the tour companies directly to book which avoids commission fees.