Website SecondWife.com promotes polygamy. This is a Sunnah but is it still applicable to modern times? Would it be okay to go ahead with this in western societies where monogamy is more traditional and accepted?
Yes, why not? Islamic rules are eternal, and they can always be practiced so long as all related conditions and terms are honored and satisfied.
However polygamy is not mandatory. It is reserved for those who desire it for honorable purposes and can properly handle it. So you don't have to practice it, and indeed, many men may not be qualified to practice it because according to Islam the man should be able to care equally for all the wives. But since in some Western countries polygamy has been criminalized, one seeking polygamy will have to find a solution for the legal obstacles if he lives in one of such countries.
As for your comparison to modern Western standards it is worthwhile to note that modern ideas of human/women rights against the traditional norms, have proven to be counterproductive and detrimental for the conditions of both women and greater society in general. As for the awful consequences of the modern Feminist movement (the main opponent of polygamy among other traditional sexual norms) I recommend this article. It explains how misguided Feminist ideas of the so-called Women's Liberation Movement coupled with the ill-intentions of the power-elite that manipulated them led to disaster for the institution of family in Western societies.
So it is important not to judge and compromise Islamic traditions based on the misguided and failed secular norms.
The website you mentioned also claims that they facilitate polygamy in strict compliance with Islamic law and ethics. They say that they oversee the users' profiles and activities and warn against any behavior in violation of Islamic code of conduct and modesty. So if that's how their site actually works, there can be hardly any objection in regards with the Islamic legality of their work. In fact it might indeed be helpful in facilitating the Islamic practice for those Muslims who need it (such as widows who may never be able to find a virgin husband and therefore have to look for married men).