For many of us of a certain generation, the thought of diagramming sentences brings back nightmares of Mrs. McGinty's fourth- and fifth-grade English. Yet it still can be useful to be able to parse out the parts of a sentence when they aren't dancing together nicely so that you can figure out what's wrong and fix it.
So I recommend Eugene R. Moutoux's Diagramming Sentences site. Yes, it may bring back some nightmares, but it's a great refresher. And if you can get down around sentence 50 and do those diagrams, you should be awarded the Order of the Grammar with Gold Palms. (And Mrs. McGinty would be so proud.)