The Pontcysyllte Aqueduct (I'm looking at it from my window now) here in Wrexham (joins the villages of Froncysyllte which is famous for it's record-selling choir and Trevor).
Built by Thomas Telford in recently been granted UNESCO World Heritage Status so it's got alot of tourism and status now (it did before, but it does even more so).
http://en.wikipedia.org/wiki/Pontcysyllte_Aqueduct