Scythe Kazemaru 2 140mm x 25mm Case Fan- 500RPM Ultra-low Speed(SM1425SL12SL)

If you need a fan that you cannot hear from a foot away, this is the one. The downside is that it really doesn't push much air. Its faster brother, the  Scythe Kazemaru 2 140mm x 25mm Case Fan- 1200RPM Medium Speed(SM1425SL12M)  is not as quiet as this model, but still quiet enough for almost any application if you run it at 5V, and at 5V it pushes a lot more air than this one.Note that this fan does not have 140mm mounting holes, so it will not fit some applications.