ChinesePod is a podcast for learning Chinese. The podcasts are free, and you can pay a little money for a subscription to access the transcripts of the podcasts, and other lesson-oriented resources. I listen to ChinesePod all the time because the lessons are so well put together. It really makes Chinese easier to learn.
Chinese-forums.com is an online community of people with an interest in Chinese language and culture.” I used their database, among others, to assemble the HSK vocabulary lists on this site.
Mandarintools.com has a lot of computer tools for Chinese, like for GB-Big5 conversion and romanization conversion (very cool). They also have a couple of dictionaries and some very good coding resources.
Mark Lu has written some good-looking flashcard software for Windows Mobile, using the HSK lists from this site! If you are on the road a lot, this could be a very useful tool. Mark has worked really hard to put it together, so email him your questions, bugfixes and feature requests. Mark also put a demo video on YouTube.
Book New Practical Chinese Reader (book 1, book 2, book 3, book 4, book 5) is the new rewritten edition of Practical Chinese Reader (books 1, 2, 3, 4) textbooks. These high-quality textbooks are used in many schools around the world. The new edition is a very well-done series.
Book Practical Chinese Grammar (link describes Mandarin Chinese grammar in a clear, concise way. This reference uses a lot of examples, and the vocabulary used in the examples is beginner-level, so you can focus on grammar principles. Using this book has really helped make Chinese grammar simple for me.