To open a file in an existing split window, drag it from the file browser to the split that you’d like it to open in.

To instantly close an open tag in Coda, hold down option, command, and period (⌘ .), or use the menu option by going to Text → Close Tag.

When in the CSS Editor mode and the “Colors” panel is open, click the magnifying glass icon to select a new color based on any part of the screen. It’ll bring up a bigger magnifying glass that, upon mouse click, will load the selected color into the “Colors” swatch.

Most think that this icon should be a color picker (and I agree), but as Panic’s Cabel notes, “This is actually a feature of the system-wide color picker in Mac OS X! You’ll get the same feature if you open, say, the color picker in Text Edit or Mail.”