Here is a guide to episodes of the original series, the animated series, the Next Generation, and Deep Space Nine. This guide has an episode list and synopsis list for each series.
Here is another guide; this has the animated series and the original series. This guide includes a lot of info besides a listing and synopsis of each episode.
This is a Next Generation guide. It also includes a lot of info and trivia besides the basic guide.
This is a original series guide, with ratings of episodes along with synopses.