Resources

MangaFox
Most manga is not commercially published in English but is translated by amateur groups (called "scanlators"). You can read many translated manga online at mangafox, although if it has been released by an English publisher it will probably be taken off the site.
I have created an account for this site so that I can bookmark manga I want to read, as well as bookmark the pages of manga I'm currently reading. I can arrange my bookmarked manga by when they were last updated or when I last accessed them, and this allows me to see which manga has new content or go back to manga that I may have forgotten about. You don't need an account to access the manga on this site, so if you are just a casual reader you probably don't need to go to the same lengths as I do.

TV Tropes
This site has all sorts of information about "tropes". I sometimes look at it when I am unsure of a concept (megane, otaku, moe... I could go on) and it does a pretty good job of explaining the cultural context. I sometimes also look for the Tropes page of a certain manga just to see what is listed about the theme or characters of the story.
*Warning* unless you have amazing willpower, set a time limit for yourself when visiting this page. Many a night's sleep have I lost because I wanted to "quickly" look something up on TV tropes....

Japanese Honorifics
I thought about doing a post about Japanese honorifics, but this TV tropes page does it so well already.
When reading translated work it is important to understand that some significance is lost in translation. Often, when  honorifics are translated into English they do not have the same effect and may even change the ambiance of the story. Honorifics play a much bigger part in Japanese language and social interaction, and it is a good idea to know at least the basic ones.