They are sometimes called wild canaries (but are in the finch family).
Their flight call is four syllables and sometimes sound like "potato-chip".
In the winter, they have a winter coat that is a duller gray-greenish-brown in color.
Click here for more information on Goldfinches, includes audio of their calls and video of the birds.
UPDATE: Spring 2010. Saw these at a lot crowded with a variety of songbirds.