Math
K - 4
Interactive math and reading activities
Free units that help students apply math to the world around them. Include units on probability, patterns and functions, fractions, decimals, percentages, data collection and analysis, and more! Though they say for grades 4-8, I would recommend for 3rd to 4th grade gifted typically.
Grade 1 Taking Shape
Grade 2 Everybody on the Move!
Collection of online math games
This is a completely free resource that allows students to learn about stocks, bonds, mutual funds, diversification, and more while playing a simulation that mimics the Stock Market exactly! They get to compete with students across your district, region, and the entire US to see who can invest $100,000 the best. There are fall and spring sessions for competition, so sign up ahead of time, teach students a few lessons in advance so they know the basics, then start competing!
K - 8
Lessons and activities for at home Engineering projects
Problems that encourage communication in math between students.
Instructional videos & activities
Looking to get kids arguing about math? Check out http://wouldyourathermath.com from John Stevens. Lots of great "Would you rather..." scenarios that require math to win the argument.
Online advanced math critical thinking games
Math activities and games
Instructional videos & activities
6 - 12
Algebra games and advanced Math
Daily Math challenges
Engineering challenges
Mathalicious - Teaching math through real world problems. Absolutely amazing site with 146 lessons for grades 6+, but costs $320 per teacher per year. However, they do have some free units that I've linked below that are good for high school.
Engaging Algebra game
Alcumus offers students a customized learning experience, adjusting to student performance to deliver appropriate problems and lessons. Alcumus is specifically designed to provide high-performing students with a challenging curriculum appropriate to their abilities.