From The Guardian, this is a do's and don'ts of gallery text and wall panel writings. It includes examples as well as things to think about as you are writing your texts.
A very thorough guide from The Getty on what to consider when creating gallery texts. Goes in depth into the creation and editing of exhibition texts. It also includes outlines, floorplans, and information on the creation of gallery graphics such as timelines and wayfinding tools.
This site provides an array of different online exhibitions from a wide range of subject areas and themes. You can filter results by museum as well as category. It also gives great examples of text to accompany online exhibitions.
This site provides an array of different online exhibitions relating to art on a number of different subject areas and themes. It doesn't give you full access to images of the exhibitions, but it does provide videos, interviews, as well as some sample images depending on the exhibition.
Provides online exhibitions of the Princeton University Art Museum. Not all exhibitions are presented in their entirety, but it does give some great examples of text to accompany an exhibition.
This site has a ton of online exhibitions available for full viewing. Many of the exhibitions include narrated tours, high resolution images, and text to accompany the works in the exhibition.
What Makes a Great Exhibition? by 
