A client wanted a glossary on her site so that keywords would link to more description of those words. She wanted some “bubble” hover over/pop-up text and/or a link to a full definition at a glossary page. I found bd’s Hover WordPress plugin and it certainly does the trick. It allows you to set up words to have hover-over text, a link to a URL, or both. You can do abbreviations, acronyms, or regular text. There are some fancy effects you can play with as well. In the following help video tutorial, I also cover using anchors (making links to a particular spot on a long page).
I have an example over on this test site.