If you're taking Microsoft Certification exams, you might be interested in the Second Shot offer that Microsoft and Prometric are running. If you take your exam between 15 September 2007 and 31 January 2008 30 June 2008, you can get a free retake on Microsoft IT professional, developer and Dynamics exams if you fail the first time.
In order to take advantage of this offer, you must register on the Microsoft website and follow specific instructions while scheduling your exam.
Info at Prometric: http://www.prometric.com/microsoft/2ndchance
Update: the Microsoft webpage for this promotion is http://www.microsoft.com/learning/mcp/offers/secondshot/default.mspx.
Update: the offer is extended till 30 June 2008!