Yale University has a great Chinese Usage Dictionary with 85 entries. Only problem is that it uses the deprecated HTML practice of frames, and the links in the left sidebar are not right. You actually can get to the articles by hovering over the links, noting the HTML file it points to, and then editing the URL in your browser, but that’s a bit tedious.
To make access easier, AllSet Learning has added an index page for Yale’s Chinese Usage Dictionary, and at the same time, added a few relevant Chinese Grammar Wiki links as well. Check it out!
The Chinese Usage Dictionary isn’t a full dictionary in the sense of Pleco or MDBG, and it doesn’t stick strictly to vocabulary or grammar, alternating between the two. But if you like comparisons of similar words with examples of correct and incorrect usage, or want some exercises, then definitely give it a look.