You are missing one of the best sites of the metaweb (or web 2.0, as some like to say): Digg. Whenever I come across an article or blog post I really like, I post it on digg ("digg it"). Unfortunately, or fortunately for everyone else, it is not geared towards engineering. There are other similar sites (Reddit comes to mind), but none that I have seen really focus on engineering or science in general. Slashdot sort of has this function, but it's typically used more for computer-related subjects, and it's moderated pretty heavily, so not everyone can post stories very easily. It would be nice to have a Digg-like website for engineering/physical sciences. The closest thing I can think of would be Faculty of 1000, but that's mainly for biology.

One thing,I recently switched from Firefox to Opera. Why? Because Firefox leaks memory, or at least, uses too much. And second, I found that Opera handles pdf documents much better.

Google book is also very useful.

Thanks for the useful list. During my study and research, I found that my maths is always not enough so I frequently connected to the MathWorld that is a free resource from Wolfram Research built with Mathematica technology. I strongly recommend this site to you.

I am a BBS manager at mechanics@newsmth. The people in this field like talk about the mechanics in our lives, such as the bridges, rains and other intresting things. We also talk about the academic problems. The people here come from the students from Tsinghua Unv., Peking Unv., Stanford Unv, and other places. We help each others solving their mechanics problems and find some useflu data for others. It is a good place to exchang the experence in learing and researching.

In addition to what Dr. Suo listed, I want to mention Google Groups . Besides many existent groups, one can easily set his/her own discussion group. For example, if you participate in an acedemic research group, Google Groups can help you building an online discussion group where people from your group can share information.