I have recently learned about “Connexions”, which is essentially an online repository of notes and learning material of different subjects, but with a lot of content focused on DSP. What makes this site so valuable is that it follows the “open access” philosophy, which means that all the information is free (but belonging to an author that has to receive credit for his/her work), and intended to spur collaboration and further contribution. Besides the learning material, this site is also interesting in how the Internet is used intelligently to promote collaboration as well as the use of new tools and technologies (such as MathML).