Economics: excellent for complex tax law and capital markets.
Engineering: project finance and construction lawyers will love you.
Any science: intellectual property law, a hot area at the moment, would be good for you. For instance biochemistry backgrounds are highly-valued among firms with strong patent litigation practices. Similarly, the growth in e-commerce and telecommunications client bases means that an understanding of new or fast-changing technologies is absolutely vital, whether with regard to IP protection or corporate work.
Politics, history, English, classics: general good analytical skills. Do not underestimate the skills you have learned here.
Languages: linguists are more or less essential for all those globalisation plans you have heard about.
NB: Even though you might think that you will be pigeon-holed by your undergraduate degree, firms and chambers generally allow you to find your own area of interest, even if it has nothing to do with your first subject.