Blender permanently increases in RAM usage after each render, whenever motion blur is used. The memory leaks are very violent, and Blender can sometimes gain nearly 1GB of RAM per render! I believe I first noticed this in Blender 2.76, but it still exists in 2.77. I use the official x64 Linux build of Blender under openSUSE Tumbleweed. Currently I need to disable motion blur so I can render my animations altogether.
I'm noticing this in camera tracks rendered under Cycles: If on the Foreground render layer I enable the Vector pass, and under composite nodes I add a Vector Blur node, the problem begins to occur. To my surprise, I noticed this also happens if I don't enable the Vector pass or use a Vector Blur node, but simply enable the native Motion Blur in Cycles (the Motion Blur option in the Render panel). In one test case, this is with how much RAM the process increased every render:
Render #1: 1.589 MB
Render #2: 2.443 MB
Render #3: 3.302 MB
There's currently one important clue I was able to uncover: The aggressivity of the leaks seems to depend on the polygon count of objects in the scene. If I disable all Subsurf modifiers, the issue lessens and sometimes disappears entirely! For the test case above, this is how enabling Simplify and setting maximum subdivisions to 0 changed the leaks:
Render #1: 1.280 MB
Render #2: 1.903 MB
Render #3: 2.526 MB
As the projects I'm experiencing this in are large (eg: video backgrounds) and currently private, I can't produce a test case I can upload as easily. The problem should be easy for others to reproduce however, unless something very specific is somehow causing it on my system.