It was Firefox which popularised the concept of plugins and now OpenOffice too has come up with nice extensions. With the OpenOffice extensions, OpenOffice.org has an edge over MS Office.
Sun Presenter Console : Makes it very easy to do presentations.
Sun Wiki Publisher : Allows you to publish directly to Wiki.
OpenOffice.org2GoogleDocs : Export your OpenOffice docs to google docs.
Get Stock Price : Pulls stock price data in realitime from yahoo finance, wonderful tool to calcuate your portfolio value in realtime.
It also has various dictionaries and more.
Download Link for all the extension.