A while ago I read a book by an American Buddhist scholar that recommended the Access to Insight Web site for its free and expertly translated texts:
These pages invite you to explore some of the Buddha’s basic teachings as they are presented in the Pali* canon. Each page in this section contains a selection of short passages from the suttas (discourses or sermons; see sutta [sutra] in the Glossary) that introduce or illustrate different aspects of a single topic. If you encounter a particularly meaningful or interesting passage you can, in most cases, read the full text of the sutta from which it came by simply following the link at the end of that passage. Many of the passages are cross-referenced to other pages, allowing you to pursue a theme to whatever depth or breadth you desire.
There is also a collection of study guides.
* Pali is the language of the texts used by the Theravada tradition of Buddhism. Sanskrit is used by the Mahayana tradition. For example, the word nirvana is Sanskrit; the Pali word is nibbana. You can get a quck overview of the difference between the two traditions at BuddhaNet.