Installing and activating TheThe Sliding Panels plugin on your WordPress site will cause activation of the following widget areas: Top Sliding Panel, Bottom Sliding Panel, Left Sliding Panel, Right Sliding Panel, and Modal Window. You can set all parameters of a panel as well as add any widget into each Sliding Panel/Modal Window widget area.
Suggest uses for Sliding panels are:
Contact us, Opt-in, Sign-Up and Log-In Forms
Alerts, Notices and Reminders
Quick Tips, Help and Support Elements
Embedding PDFs, Videos and downloads
Image: screenshot of a Modal Window
Download and info: http://wordpress.org/extend/plugins/thethe-sliding-panels/