XKCD Hovertext for Chrome
More About XKCD Hovertext
The appearance and behavior of the box is highly customizable (check out the options page for the extension).
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