catalin.m

With this plugin you can create multiple galleries, all filed under “Gallery” page. 
Very easy to use, just install, and start adding galleries in your Gallery section in admin. 
Pictures are added in each gallery with the help of WordPress’s Media Uploader. If you want a certain image to show up as the gallery cover instead a random one, set it as featured image. You can select two sizes of the images: one size for gallery cover images, another for thumbnails shown in each gallery page.

AW Gallery

Create multiple galleries, each with it's own page.

Yannick Lefebvre

The purpose of this plugin is to allow users to create one or more modal dialog(s) / pop-up window(s) that will appear when a user visits their site. The number of times that these can load is configurable. They can load content from external sites or custom HTML code into the dialog. This plugin can be used to invite people to register to a newsletter, respond to a survey, or simply welcome come to a site upon their first visit.
You can try it out in a temporary copy of WordPress <a href="https://demo.tastewp.com/modal-dialog" rel="nofollow ugc">here</a>.

Modal Dialog

The others either didn't work or caused massive JavaScript problems, breaking other plugins.

This plugin worked the best.

Worked better than the others

The purpose of this plugin is to allow users to create one or more modal dialog(s) / pop-up window(s) that will appear when a user visits their site.

AW Gallery vs Modal Dialog