What sauce could I use for haddock?
Compound butter is a very nice accompaniment for fish. 

"Compound butter is a mixture of softened butter and at least one other ingredient.  Some ideas for compound butter are: herbs, spices, citrus zest/juice, honey, fresh mint, soft cheeses (like goat), fresh or dried peppers, and nuts…you get the idea – this list could be endless!"

You can cook the fish on its own and then put the compound butter on top to melt over it. While soft the butter can be put into a piping bag and piped as rosettes and refrigerated until ready to use. Makes for a nice presentation. 

Another common accompaniment to fish is mango salso or a combination of tropical fruits (mango pineapple for example). 

This seemed like a good link for ideas for sauces for Haddock:  

http://www.wisegeek.com/what-are-different-types-of-sauce-for-haddock.htm