My personal favourite is http://www.thecolor.com/ It's very colourful and has plenty of pictures to colour ranging from Aesop's Fables to Zodiac signs. The number of categories is amazing but the pictures are pretty basic. Perfect choice for kindergarten and grade 1 who don't really know how tu use a mouse yet :)
For online coloring, you can also try http://www.myfreecolouringpages.com/ This website also offers printable coloring pages, dot-to-dot exercises and an oline picture maker. The last one is a LOT of fun!
On the http://www.thekidzpage.com/ we can find games, clipart, learning pages, puzzles and more. I personally reccommend the Fun Learning Games. The Math Flash Card Game was really cool and your students will surely like it too.
http://www.coloring.com/ does not really look appealing but has plenty of coloring pages to bring into play.