Tafsir Ibn Kathir is one of the greatest commentaries of the Quran – and possibly the most famous one.
While I’d advise everyone to buy the Darussalam translation, here’s a website that will suffice for the moment: http://www.tafsir.com/
Update: The above website seems to be down. Here’s another one: http://www.qtafsir.com/
Note: I believe there are a few mistakes in the Arabic so please recheck before you copy and paste. Baarakallahu feekum.