There's no such functionality in Blender. 😉

Oh noo.. 😦 Anyway, thx for the fast answer, Sim533! 😉

You can actually render only part of your image and reassemble the pieces. Not ideal but better than nothing.
http://www.mecabricks.com/en/forum/topic/79

When you click on the render button, only the area selected will be rendered.

Hmmm, great idea! I will try it!

Or you can use the little workaround describe here : http://www.elysiun.com/forum/showthread.php?256212-Workaround-solution-for-pausing-renders-in-Cycles

Blender really need a pause and resume button. A lot of engine who works on the same principe does it (Luxrender, Octane...) I really hope to see this feature soon...

Thanks SaitoGray!

On Windows 7+ you can simply accomplish this as follows:

• Open the Windows Task Manager
• Switch to the Performance tab
• Click on the Resource Monitor button
• Select the process blender-app.exe from the list
• Right-click the entry and select Suspend Process

Right-click it again and select Resume Process to continue rendering.

Another option would be using PsSuspend. This would allow you controlling that via batch files.

If you just would like to be able to continue browsing or the like while rendering, switch to the Process tab in the Task Manager, right-click blender-app.exe and assign a lower priority. Lower than normal works fine for me.

Wow, great, thank you wlange! 😃

Yet another option would be using Process Lasso. It allows you to restrain the resources certain applications use. For example, if you've got a multi-core processor, you can limit the number of processor cores a hungry application claims for itself, so that your system remains accessible. Based on your configuration, the assignment of resources can happen automatically in the background, so that you don't have to worry again about your system becoming unresponsive as soon as you kick off a Blender render.