KidsClick resources were selected by librarians. The MarcoPolo Website bills itself as Internet Content for the Classroom. In fact, it is just that. EconEdLink for economics and Illuminations for mathematics. Each site has lesson plans that are searchable by topic as well as grade level.
Most lesson plans are well organized, complete, and cross-curricular.
There are also other web links provided from within each site. Each room along the hallway has a different activity. In the Science room, I made secret messages, in another room, I wrote poetry. Some of these might be great sponge activites for starting a lesson. Use them with the computer hooked up to the television or the projector.