Other plugins on wordpress.org:

• Faceted Search Widget
• Facetious
• Taxonomy Picker
• Taxonomy Widget

Libraries:

• wp-taxonomy-filter

Random bits of code:

• http://wordpress.org/support/topic/searching-with-multiple-tags-using-checkboxes