Here's a handy bookmarklet I use that let's me select text on a web page, then click the bookmarklet to look it up directly on Wikipedia. You may find it useful.

Find on Wikipedia

javascript:u='http://en.wikipedia.org/wiki/';s='';d=document;w=window;try{if(d.selection){s=d.selection.createRange().text;}else if(w.getSelection){s=w.getSelection();}else if(d.getSelection){s=d.getSelection();}location.href=u+encodeURIComponent(s);}catch(e){location.href=u;}

You can even update the u= URL to look things up on other sites, like Google, Google News, etc. Here are some other versions:

Find on Google Web

javascript:u='http://www.google.com/search?hl=en&q=';s='';d=document;w=window;try{if(d.selection){s=d.selection.createRange().text;}else if(w.getSelection){s=w.getSelection();}else if(d.getSelection){s=d.getSelection();}location.href=u+encodeURIComponent(s);}catch(e){location.href=u;}

Find on Google News

javascript:u='http://news.google.com/news?hl=en&tab=wn&ned=&q=';s='';d=document;w=window;try{if(d.selection){s=d.selection.createRange().text;}else if(w.getSelection){s=w.getSelection();}else if(d.getSelection){s=d.getSelection();}location.href=u+encodeURIComponent(s);}catch(e){location.href=u;}

Find on Google Images

javascript:u='http://images.google.com/images?hl=en&q=';s='';d=document;w=window;try{if(d.selection){s=d.selection.createRange().text;}else if(w.getSelection){s=w.getSelection();}else if(d.getSelection){s=d.getSelection();}location.href=u+encodeURIComponent(s);}catch(e){location.href=u;}

Find on Google Books

javascript:u='http://books.google.com/books?q=';s='';d=document;w=window;try{if(d.selection){s=d.selection.createRange().text;}else if(w.getSelection){s=w.getSelection();}else if(d.getSelection){s=d.getSelection();}location.href=u+encodeURIComponent(s);}catch(e){location.href=u;}

Find on Yahoo Web

javascript:u='http://search.yahoo.com/search?y=Search&ei=UTF-8&p=';s='';d=document;w=window;try{if(d.selection){s=d.selection.createRange().text;}else if(w.getSelection){s=w.getSelection();}else if(d.getSelection){s=d.getSelection();}location.href=u+encodeURIComponent(s);}catch(e){location.href=u;}

Find on Yahoo News

javascript:u='http://news.search.yahoo.com/news/search?c=yahoo_news&p=';s='';d=document;w=window;try{if(d.selection){s=d.selection.createRange().text;}else if(w.getSelection){s=w.getSelection();}else if(d.getSelection){s=d.getSelection();}location.href=u+encodeURIComponent(s);}catch(e){location.href=u;}

Find on Bing Web

javascript:u='http://www.bing.com/search?q=';s='';d=document;w=window;try{if(d.selection){s=d.selection.createRange().text;}else if(w.getSelection){s=w.getSelection();}else if(d.getSelection){s=d.getSelection();}location.href=u+encodeURIComponent(s);}catch(e){location.href=u;}

Find on Bing News

javascript:u='http://www.bing.com/news/search?q=';s='';d=document;w=window;try{if(d.selection){s=d.selection.createRange().text;}else if(w.getSelection){s=w.getSelection();}else if(d.getSelection){s=d.getSelection();}location.href=u+encodeURIComponent(s);}catch(e){location.href=u;}

Find on Twitter

javascript:u='http://twitter.com/#search?q=';s='';d=document;w=window;try{if(d.selection){s=d.selection.createRange().text;}else if(w.getSelection){s=w.getSelection();}else if(d.getSelection){s=d.getSelection();}location.href=u+encodeURIComponent(s);}catch(e){location.href=u;}