Definitely, SMART Exchange! I love to use my Smart software, and SMART exchange makes it even easier with so many pre-made lessons and ideas. They are interactive, so I can change anything I need to fit my lesson and my students.
Illuminations is another site I like: http://illuminations.nctm.org/Lessons.aspx?grade=3&standard=5 Lots of great ideas, lessons, games, etc. for every subject.