XKCD Hovertext

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This extension replaces native browser title text with a customizable popup for xkcd images.This extension allows you to view xkcd mouse over text in a hover box instead of the normal built-in tooltip (which generally doesn't stay up very long and doesn't look nice, etc).

The appearance and behavior of the box is highly customizable (check out the options page for the extension).

Enjoy!

UPDATE:
Version 1.6.1: update manifest to support https
Version 1.6: add support for customizing font and whether the text is "small-caps" or normal
Version 1.5: upgraded to manifest v2; added what-if.xkcd.com support; removed Google reader support (since it has been discontinued)
Version 1.4 fixes bugs caused by new page layout on xkcd.com
Version 1.3 now supports Google Reader