Extensions that start with the letter F, require jQuery for the Control Panel, are Free and are sorted by Rating (descending).
Field General lets you assign multiple field groups to any weblog/channel. This lets you reuse and share sets of fields, rather than having to duplicate them for each channel.
ExpressionEngine extension for rapid template development with AJAX template saving.
Flickr for ExpressionEngine happily marries Flickr and your EE-powered website. The add-on gives you several options for integrating Flickr - a Flickr Fieldtype, a handful of template tags, and Wygwam/Matrix compatibility.