Kaspars

Use PHP output buffering to extract widget output and store it into WordPress transients for faster retrieval. It also adds a checkbox to widget controls to exclude it from being cached.
It is a quick fix for bad behaving plugins that parse RSS feeds or call remote URLs on every page load.

Widget Output Cache

Improve website performance by caching widget output in WordPress transients.

Menkii

Removes line-breaks, tabs, double space, html comments and wordpress head-data from html output.

Output Optimizer

Removes line-breaks, tabs, double space, html comments and wordpress head-data from html output.

Widget Output Cache vs Output Optimizer