Say I have two windows of Google Chrome, each with multiple tabs. Is there a way to merge the two windows (other than manually dragging the tabs one at a time from one window to the other)?
The Windows To Tabs extension for Chrome will do the trick.
You simply hit a button in Chrome and all tabs from any open windows are merged into one window.
In fact, there are loads of extensions out there, like JoinTabs which gives you more control over what windows are merged into what other windows.
This functionality now exists in Chrome and has since version 24 or earlier. - So no need for extensions (any more)
Left-click the first tab in the window you want to merge, then hold the shift key and left-click the right most tab. All of the selected tabs for the current window can now be dragged and dropped as as group on another Chrome window.
One of my favorite extensions (certainly the one I use the most) is Session Buddy. It's so incredibly useful and merging windows is just one of it's many many features.