var rows = document.getElementsByTagName("table")[0].rows;
var last = rows[rows.length - 1];
var cell = last.cells[0];
var value = cell.innerHTML
Try it yourself here: http://jsfiddle.net/ReyNx/.
Obviously you'll have to change document.getElementsByTagName("table")[0]
to appropriately match your table
If you're using jQuery it's easier:
var value = $('table tr:last td').text();
For more info, see the MDN DOM reference, which shows you which properties are available on which elements to traverse the DOM.