All of these sites have an interactive component to them that makes them a nice tool to use to get students involved using the Mimio or any other Interactive White Board. As with any activity or tool, some resources might fit your needs better than others and some resources lend themselves to higher order thinking skills better than others. Also don't forget about curriculum materials from manufacturers that might also be available to you too.
BBC Bitesize - (K-3) This site lets you explore different interactives with Reading, Math, and Science.
EdHeads - (2-5) This site contains 10 different modules around Science themes. The modules are well done and provide interactive components along with teacher guides and other materials.
Funbrain - (K-5) This site includes many games, interactive activities, and flash cards.
Illuminations - (K-5) This site is by NCTM and has tons of Math activities and manipulatives that are aligned with national standards. This has quality resources that are searchable by grade level.
Internet4Classrooms - (K-5) This site is searchable by grade level and also subjects. It organizes connections to many good interactive sites. It includes skill building and skill reinforcement activities.
Kent ICT - (K-2) This site has many interactive games related to Literacy and Math.
KerPoof - (K-2) This site helps with Literacy skills. The interactives are around making a movie, card, word, story, or picture.,
Kids' Zone - (K-5) Within Kids' Zone, there is a section called "Create a Graph". Use this easy template to display data visually.
PhET - (1-5) This site has many very good interactive simulations related to Science and Math. The site is searchable by grade level as well as subject. It is a wonderful resource.
ReadWriteThink Interactives - (K-5) There are tons of searchable interactives related to Literacy. This is a quality resource which is part of Thinkfinity.
SpellCity - (K-5) This sites lets you create customizable lists of spelling words and save them. From the lists, you can create games, auditory reinforcements, and practice activities.
StarFall - (K-2) This site has a number of interactives around Literacy. It's focus is on reading, fluency, and decoding.
Virtual Manipulatives Library - (K-5) This site has interactive manipulatives indexed by grade level and content strand within Math.
Word Shapes - (K-2) This site lets you create word shape boxes that then can be used interactively within the Notebook software to use the ink pen on them.