Update on February 10, 2015: Oh no! nciku.com is no longer online as of the end of 2014. The article below has been left for archive purposes and this resource has been removed from our directory.
nciku is a sweet tool for learning Mandarin Chinese online which encourages collective user activity to create a wealth of educational material.
Of special note at nciku.com is the handwriting recognition tool, which allows you to write in characters with your mouse. Using this, you can further benefit from their radical dictionary, which provides audio samples in addition to defining the radicals and showing how their various forms are written. Sorting of radicals by stroke number is also available.
As nciku grows, their conversations, Q&A and theme words sections will surely expand into a considerable, lively and up-to-date resource for the many subtleties of learning Mandarin.
The nciku blog is quite informative, keeping you up to date on happenings at nciku.com as well as tidbits about everyday usage of Mandarin Chinese, such as this article about saying "excuse me" while navigating through a crowd.
nciku also offers a mobile version of their site.