Another link dump - and to make it even less thoughtful, I am linking to someone who has clearly put effort into preparing a list of useful links.
Shelly Terrell at Teacher Reboot Camp offers a list of 20+ resources to help children draw and create art, and also to compare and contrast art.
She doesn't mention Aviary, which has some excellent tools online for image manipulation and for audio and possibly video.
Well, I'm at it, let me compliment Google Docs for their relatively new drawing tool online.
Joachim Thornstrom (with an umlaut over the second "o" in Thornstrom) produced a Prezi in what I guess is a Scandinavian language but most of the brand-names are understandable and name interesting web tools online. His Prezi is here and although it lacks links, it names some useful sites for a variety of creative uses.
--Screenr - a sort of video screen capture website that allows you to make videos of what's on your screen up to five minutes long. I love it.
--Sound Bible - holds a variety of public domain sound effects in mp3 format.
--Myna Audio editor - ah, part of Aviary
--Glogster - I think it is a poster making website.
--Audacity - a free mp3 making program you can download to record yourself with. This program you download, while Aviary's is entirely in the cloud. I used Audacity when I used a Windows computer, but haven't needed it since switching to Mac. - I am not a Macophile, but I do like Garageband!
--Roc Music creator - I suspect this is part of Aviary as well.
--+ several more.
Since I started with Terrell's list of links, I should finish with this Prezi on engaging web 2.0 tools by Ryan McCallum - It is more focused on teaching than on creativity per se, but does provide a good bookend.