Below are some websites that focus on science in one way or another. Some are for homework/conceptual help, some offer interesting articles and topics in emerging science.
Physics Buzz is a blog of science news from all around the world. Sometimes, it has to do with physics, but other times, it brings in chemistry, biology, astronomy, meteorology, and pretty much any other "-ology" you can think of. Posts are archived and you can search the blog by keyword. Great resource for current science events.
The Khan Academy
The Khan Academy is a website of tutorial videos produced and maintained by Salman Khan, an independent educator who has produced 1600+ videos so far. All videos are completely free and are hosted on YouTube, so access is universal. All videos are organized by area of study (algebra, trig, chemistry, etc).
Adrian Dingle's Chemistry Pages
Adrian Dingle is one of the most accomplished AP Chemistry teachers I have ever come across. Adrian has developed an AP Chemistry program for sophomores that have never taken chemistry before. His website is a treasure-trove of information on the AP exam topics covered, including blogs on breaking College Board news as well as statistics on his student's performance each year (on the 2010 exam, 31 out of 34 students achieved a score of '5' with the other three students receiving a '4').
One great resource are his "Pop-Up" AP practice questions for each topic that connect to real free-response questions from past exams. These are a great study tool leading up to different chapter exams as well as the AP exam in May.
Run by ChemAxon, their goal is to provide an online catalog of chemical structure images (gifs, jpegs) as well as properties about most organic and inorganic chemicals. Totally free to use and with a search function. This site is still in its beta form, but hopefully reviews and surveys submitted by users will improve functionality and usability quickly. Best functionality is in Firefox, Safari, or IE.
Jonathan Bergmann and Aaron Sams kickstarted the flipped method being used around the world. This is a website that hosts all of their class videos plus videos for most other subjects, including AP Biology, Astronomy, and AP Physics. This website is up and running, but they are continually adding new subjects and will soon include English (literature and writing) and foreign languages.