Dannager
First Post
D&D Insider : This is a paid subscription service from WotC, you pay a monthly fee and get access to a Character Builder that AUTOMATICALY produces a PDF character sheet that gives you printable Power Cards, where all the abilities for your character are included (What you need to roll, what it is against, what benefits it has etc). D&D Insider also includes D&D Magazines (with new adventures and class modifications) as well as the Rules Compendium, which is an update for any rule that exists... This is useful if a question about a certain power arises, stick it into DDI and it pumps out information on it.
I'll note that the real utility of the Compendium is not necessarily as a quick reference during gameplay, but rather as a tool for creating your own adventures/encounters. Being able to say, "Okay, I want all undead elite creatures of level 5-7 from any official source in the entirety of the game," and having it hand them all to you on a silver platter is amazing.