I really owe a lot of my progress with Mandarin to Google and Mozilla. I already wrote about the amazing Mozilla add-on, but I thought I would mention two more web features that have helped me.
The first is a Mandarin tab for Google. Some time back, a group of people created a tab that links to several Mandarin resources. You can personalize the page by tweaking the settings, and adding/removing sites. Since iGoogle is one of my home pages (my browser opens two tabs at the start), I always have characters and new lessons staring me in the face. This makes me feel like I never completely neglect Mandarin 
You can add the tab to your Google page here.
The second resource I use, and have found to be very useful is not all that special: Google Reader. I follow a lot of language blogs, and I don’t think I would be able to stay so up-to-date if I did not have them all in one simple reader. If you don’t use Google Reader, you should check it out. I don’t recommend adding too many sites (if can be overwhelming), but you can always add and delete as you go.
