I'm trying to determine what the new options are available for a character who has so far been managed through the offline character builder. I've noticed that any of the Essentials books and Heroes of Shadow are new sources not previously available, but which magazine issues are new since the final offline CB update?
I just booted up my old offline character builder for this as I, too, was curious.
If you boot up the builder and click to create a new character, followed by going to "Manage" and "Campaign Settings" you will find a complete listing of resources used in the builder itself.
The most recent issues of Dragon and Dungeon magazines are issues 390 and 181 respectively. These two magazines have the "Beta" tag next to them, but other than that, it appears that all available content from those magazines is available for building.
Note: The only other thing listed in beta is Psionic Power
Here's a quotation taken from the email I got about the release of the web-based character builder:
On November 16 , Wizards of the Coast will launch a brand new web-based version of the D&D Insider Character Builder. The current downloadable version will remain functional, but will no longer be available for download or updated with content after the new version goes live.
The November 2010 issue of Dragon Magazine is #393.
The November 2010 issue of Dungeon Magazine is #184.
I'm not sure if they actually included the content from the most recent magazine as of November 2010. You may be safer going back an edition or two.