By Mary from MI
Since you haven't posted a picture it's hard to answer this one, but I'll give it a stab and suggest it might be an agave. Have a look at this Google images search result to see if it looks like your plant:
http://www.google.co.uk/search?tbm= ... 1764l0l4460l5l5l0l1l1l0l77l295l4l4l0
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