Popupper is a plugin that enables a blogger to add popups of images and text into their posts.
Any text that you select in a post or a page can be turned into a popup.
The text is decorated in someway, you can choose the text to be underlined, double-underlined, boxed, or highlighted with various colors.
The button stays gray in the WYSIWYG editor until you select text that you wish to use for the anchor of the popup. The anchor of the popup is the text that triggers the popup when the reader mouses over it.
Currently, there is not a good way to choose the height and width of the popup, it is chosen automatically. Future work would be to allow this to be customized.
In addition, once a popup is created there is not a good way to go back and edit the popup, instead you need to delete the old one and create a new one. Future work is planned to address this.
WordPress 2.5 up to WordPress 2.7 Beta 3, as soon as there is a newer beta, I'll test that out too.