Bookmarklet for The Free Dictionary (drag this to your Bookmarks Toolbar and when you click on it you can enter a word to look up its definition):
Bookmarklet
In raw code:
javascript:var%20q=escape(window.getSelection()),i,ii;if(!q){for(i=0;i<frames.length;i++){var%20fr=frames[i];try{q=escape(fr.getSelection())}catch(e){};if(q)break;else{for(ii=0;ii<fr.frames.length;ii++){try{q=escape(fr.frames[ii].getSelection())}catch(e){};if(q)break;}}}}if(!q)void(q=prompt('Enter%20word%20to%20define%3A',''));if(q)void(location.href='http://www.thefreedictionary.com/'+q);