my suggetion,

There are really 3 major ways to say this in English, draught like Isabel has stated and then you have draft and tap beer. I personally say 'what's on tap?' or 'what beer is on tap?'...

The first Old Chicago Restaurant on Pearl Street in Boulder, Colo., seemed exotic a dozen years ago, when it offered more than 20 beers on tap. ...
www.beertravelers.com/lists/multis.html - Em cache - Semelhante

A Guide to taverns, inns and pubs in the Greater Philadelphia area that serve local beer on tap.
philadelphia.about.com/od/.../a/taverns_3.htm - Em cache - Semelhante

Beer on Tap. Fotografia stock | Ficheiro #: 8323540 Fotógrafo Exclusivo da iStockphoto. Voltar para resultados. Detalhes da fotografia Stock ...
portugues.istockphoto.com/stock-photo-8323540-beer-on-tap.php - Em cache - Semelhante

I found 12,300,000 google searches on beer on tap

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Note added at 19 mins (2009-07-17 09:13:58 GMT)
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This also depends on the context, if it's a conversation, a statement or a list of beers available on 'tap'...what it comes down to is popularity and what region you are talking about and also dif. English speaking countries have their own preference when refering to this...good luck!

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Note added at 4 days (2009-07-22 08:52:37 GMT) Post-grading
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You are very welcome, have a great day!