The National Learning Network have made available a host of learning resources. These were funded by the LSC and are available free to all authorised organisations in the post-16 sector.
They describe the resources as "small, flexible 'bite-sized' episodes of learning". Designed to be fun, the activities can be attempted over and over again and cover many different topic areas (including core skills, mainstream school subjects and vocational areas). These are definitely worth looking at if you want online activities for your students to increase their knowledge and to check their understanding.