While there's no substitute for visiting a real art gallery, there are now a vast array of resources online for getting an idea of different artists' styles, and a glimpse of some of the artistic treasures of the world. Some of these are listed below.
National gallery UK - see some of the collections online, by theme or by period; read biographies of some famous artists; find background information to classical and mythological references in some of the pictures.
(Note that most major art galleries in the UK and elsewhere have online resources - try searching by name, or for 'art galleries').
Monet's art - Links to anything related to Claude Monet, including his gardens at Giverny and lots of paintings. Suitable for all ages.
Art attack - based on the British TV show, this site has some excellent ideas for different art styles, with simple explanations and examples, for children of all ages.
Download-a-dinosaur - create your own dinosaurs from paper, following the instructions. You need paper, a printer, scissors and glue.
Art junction - mostly intended for teachers, this is a big site with ideas for helping children learn to be artistic, a gallery of children's artwork, and some suggested art projects. Well worth browsing.
Paper craft - download and print hundreds of designs, with instructions online, to create a wide variety of paper objects - animals, vehicles, national costumes, buildings around the world and much more. Something for all agegroups.
Papier mache - excellent and thorough site explaining all that is needed for papier mache models, with several tutorials as well as some articles.
Kinder art - American site intended for home educating parents who want to teach art and craft formally. Hundreds of 'lessons' listing required resources for various projects - plenty of ideas, and they can always be presented informally!
Art studio - explanations about perspective, atmosphere, shading and other techniques. Ideal for teenage artists wanting to learn more.
Figure drawing - detailed and clear explanations of how to draw figures and portraits. Excellent resource for teenagers and adults.
Art Rage - excellent graphic art program which my older son uses extensively.
Blender 3d graphics - my sons' favourite 3d modelling program. The program is freeware and downloadable from this site. Galleries show the kind of artwork that can be produced using Blender.
Art and craft - English - Geography - History - Languages - Maths - Music - RE - Science