jQuery Lightbox For Native Galleries (jquery-lightbox-for-native-galleries) free WordPress plugin

DISCONTINUATION NOTICE: Development on this plugin has ceased in favor of the carousel module in the Jetpack plugin. I recommend you use that plugin instead — it’s much more full featured.

Makes the native WordPress galleries use a lightbox script called ColorBox to display the fullsize images right there in the page. No modifications required.

You can also make single non-[gallery] images use the lightbox by giving their links a class of “lightbox”, like so:

<a href="..." class="lightbox"><img src="..." alt="..." /></a>