Basic chess problems -- I use them as warm-ups for chess club.
A huge site with lots of features. Has a large user base for live play via a computer, as well as correspondence play via the Internet. Includes training videos, tactics training, computer based lessons, and an active user community. You need to subscribe to get the full benefits of what there is here, but there are lots of free resources, too.
Database of chess games. Lots of search options, the ability to step through the games, plus PGN files of the games are available. There is also a community of members who can add commentary to the games.