an useful vs a useful

Some examples from the web:

• The possibility of extending the duration of an auxiliary contract constitutes an useful element of flexibility in the use of the institutions' human resources.
• The GEGL Operation tool has been added in GIMP 2.6 and was originally meant as an useful experimental tool for GIMP developers. The GEGL Operation tool enables applying GEGL operations to the image and gives on-canvas previews of the results.
• And they're an enormously useful tool in modern molecular biology.
• As for the Otila report it deals with an extremely useful aid measure.
• That is an extremely useful definition.
• Exchange of best practice has proved an especially useful tool.
• The Internet is undoubtedly an extremely useful communication tool, which can however be misused and exploited by terrorist organisations.
• That was an exceptionally useful piece of political coordination.
• The land has an unlimited useful life.
• In conclusion, your rapporteur sees Ecolabel as an extremely useful tool for improving the environment-friendliness of European products and stimulating research and technological innovation.
• The draft report is an extremely useful compendium of data and commentary that is not otherwise available in readily consultable form.
• But the setting up of this Tribunal persuaded me that it is an extremely useful instrument for international peace and for preventing other large-scale crimes.
• I would like to congratulate Mr McMillan-Scott on an extremely useful and balanced report on relations with China.
• This year was, in my opinion, an extremely useful one in the field of justice and home affairs.
• Since then, Parliament's Directorate-General for Research has produced an extremely useful update study.
• This is an extremely useful remedy for the citizens, businesses and other organisations throughout Europe, confronted as they are with cases of maladministration within the European institutions.
• Late last month, I paid an extremely useful working visit to Kosovo, where the situation is alarming.
• We are living through difficult times but we have in our hands an extraordinarily useful instrument to tackle these difficulties.
• The new Treaty is an unquestionably useful tool for the EU, but it cannot be used to establish a multi-speed Europe.
• Mr President, this has been an extremely useful and valuable debate for the preparations for the summit.

Some examples from the web:

• The Model NWC was outlined as a useful document to assist in such negotiations.
• - Always a useful thing to have along, sir.
• Life with a hypochondriac mother gives you a useful knowledge of pharmaceuticals.
• We have a useful tool, we must use it to improve coordination.
• - A camera's a useful gadget.
• Quite a useful tool in a seduction.
• It played a useful, even essential role.
• Benchmarking can be a useful instrument for keeping track.
• Absolute control is a useful thing.
• The Irish proposals provide a useful platform here.
• The Amsterdam Conference produced a useful but incomplete result.
• Regional policy is a useful tool for islands.
• Your plain speaking everyman perspective is a useful counterpoint.
• Could be a useful positioning piece for me.
• All right, here's a useful lesson for you.
• I thought he might make a useful addition to our team.
• This business serves as a useful front.
• My mom told us to make a useful life.
• Study hard and be a useful person in the future.
• Consultation of a credit database is a useful element in the assessment of creditworthiness.