I'm planning to travel with my best friend this summer and I was wondering if any of you guys know a website that list low cost airfare for each country?
Unfortunatly it's a bit outdated, but used to be a good starting point. Most of the routes still apply. It's possible that there are new ones that are not listed yet.
In any case keep in mind that you are looking for cheap prices, not for low cot companies. It's not uncommon that a traditional company can provide a cheaper price in specific conditions or a cheaper solution (eg. the ticket might be more expensive but you land in a near airport to destination therefor reducing transfer costs). I would give a try on a fare search engine like skyscanner, momondo or kayak. Each has its own strengths. In skyscanner, for example, you can even do searches like cheapest prices for a given month or to a given country, instead of a city in case that suits you. You will get prices for both low cost and traditional companies and you will be able to compare the best solution.
Ryanair used to be outside these results since they didn't allow fare search engines to get their info. You would need to go directly to their website. Currently that no longer happens. I am not aware if there are other companies with the same policy, that are worth looking directly on their website.