Typo fix: usefull -> useful

doc/cifarSC2011/extending_theano.txt

@@ -79,7 +79,7 @@ and related methods allow the op to generate c code that will be
 compiled and linked by Theano. On the other hand, the ``make_thunk``
 method will be called only once during compilation and should generate
 a ``thunk``: a standalone function that when called will do the wanted computations.
-This is usefull if you want to generate code and compile it yourself. For
+This is useful if you want to generate code and compile it yourself. For
 example, this allows you to use PyCUDA to compile gpu code.
 
 Also there are 2 methods that are highly recommended to be implemented. They are

doc/internal/dev_start_guide.txt

@@ -85,7 +85,7 @@ branch and hit the "Pull Request" button in the top right corner.
 If you don't get any feedback, bug us on the theano-dev mailing list.
 
 When the your pull request have been merged, you can delete the branch
-from the github list of branch. That is usefull to don't have too many
+from the github list of branch. That is useful to don't have too many
 that stay there!
 
 .. code-block:: bash

theano/sandbox/cuda/basic_ops.py

@@ -2098,7 +2098,7 @@ def profile_printer(fct_name, compile_time, fct_call_time, fct_call,
     if any([x[1].op.__class__.__name__.lower().startswith("gpu") for x in apply_time.keys()]):
         local_time = sum(apply_time.values())
         print
-        print 'Some info usefull for gpu:'
+        print 'Some info useful for gpu:'
 
         cpu=0
         gpu=0

theano/tensor/tests/test_blas.py

@@ -1040,7 +1040,7 @@ class BaseGemv(object):
         # The only op in the graph is a dot.
         # In the gemm case, we create a dot22 for that case
         # There is no dot21.
-        # Creating one is not usefull as this is not faster(in fact it would be slower!
+        # Creating one is not useful as this is not faster(in fact it would be slower!
         # as more code would be in python, numpy.dot will call gemv itself)
         # See ticket 594
         """