Zotero is a free, fairly easy-to-use Firefox extension to help you collect, manage, and cite your research sources.
It's a good tool for doing literature review. for example (as a personal and light-weight alternative to wikis like this one). It also may be considered a personal learning environment, under the condition that the user finds an appropriate organization of the library...
Zotero is a browser extension. In our opinion, the potential "market" for such extensions is huge in education. An other example is for example the eLML editor (beta in 2009).
Also, we consider it important that students (in particular in educational technology) are aware of such extensions and also of user-side JavaScript.
Note: Editors like eXe are not extensions, but use the application framework of a web browser.