Here is a great 'mashup' (use of two or more established net technologies or tools to create a new service). This related images tool by GoogleBlogoscoped creates a 'set' of related images when you put in a search term. Enter 'cow' and you'll get a number of images of animals. This is a great way to create a visual dictionary for yourself or class.

Here on the site, there are four visual dictionaries that provide pictures and example sentences:

Sports
Food
Animals
Professions