ExVisited is a script written by Smoer that was originally posted to an imageboard.
The script has been slightly updated and has been uploaded here for greater ease of install and visibility.
If you already have ExVisited installed and want to move your data from the http version of ExHentai to the https version of ExHentai, then follow this guide: http://pastebin.com/dsS165B4
You can skip the steps listed for editing the script if you install this version over the one you have currently installed.

FEATURES:

• Allows the user to see which galleries have been visited by changing the color of the gallery in both list and thumbnail view. This color can be changed by using the CSS function shown in the images.
• Shows the user the time at which the gallery was last visited in both list and thumbnail view.
• Provides a method to hide individual galleries by clicking the red "X" by the gallery's name while in list view.
• Gallery data can be exported and imported.

LIMITATIONS:

• Does not keep gallery data between both sites. The user will have to manually transfer the data from e-hentai to exhentai or vice versa to have it be on both sites. The user must export the data from one site then import it on the other to accomplish this.

• The CSS color used to distinguish visited galleries is shared between sites. As the default color is not suitable for e-hentai, I suggest clicking on the CSS option and changing the value from #222 to #c96.

• After a gallery has been visited or hidden, there is no simple way to unvisit or unhide them. This can be done by editing the local storage file in which the data is contained.

UPDATES FROM PREVIOUS VERSION:
Lines added to allow functionality to the sites listed below. Previously HTTPs exhentai and e-hentai were not supported.

// @include     https://exhentai.org/*
// @include     http://g.e-hentai.org/*
// @include     https://g.e-hentai.org/*
// @description Shows when a gallery was last visited and allows the hiding of individual galleries

Version changed from 2.0 to 2.1

NOTE:
I will not be able provide support aside from what is listed here or fix anything major that breaks.
If anyone can edit this script to where the data is shared between the sites, that'd be great.
Another feature would be allow to hide galleries in thumbnail.
Does not show a gallery as already viewed after it's been updated.
Being able to revert changes would be nice as well.

For example images, look here.