The 'Widget logic' field at work in standard widgets.
The widget_content filter and wp_reset_query options are at the foot of the widget admin page. (Both are off by default.)
1More
Steps to Develop Custom Widget in WordPress - 0 views
1More
WordPress › Swifty Image Widget « WordPress Plugins - 0 views
1More
How to Add Widgets to WordPress Theme's Footer | Tips and Tricks HQ - 0 views
1More