Flags with CSS3 Gradients

Bangladesh
Red round circle with a green background
Belgium (via @mathias)
Black yellow and red vertical stripes
Cuba
Blue and White horizontal stripes, with left red trianble and white star
Denmark
Offset white cross on red
Estonia
Black, Blue, and White horizontal stripes
France
Blue, White and Red vertical stripe
Greece
9 blue and white stripes, with a cross in the top left
Holland / Netherlands
Blue, White and Red horizontal stripe
Finland
Offset blue cross on white
Gambia
red, blue and green horizontal stripes with white separators
Ireland
Orange, White and Green vertical stripe
Japan
Red round circle with a white background
Liberia
13 red and white stripes with a white star on blue background
Libya
red, black and green stripes with a star and crescent
Kuwait
green, black, red and white
Norway (via Jan Henrik Helmers)
Blue and white offset cross on red background
Russia
White blue and red horizontal stripe
Sweden
Offset yellow cross on blue
Spain (via Minerva Juan)
The union jack
United Kingdom
The union jack