Sizer is a free tool which resizes desktop windows to precise dimensions, ideal when you need a 1024x768 (or whatever) screenshot and don't want to resize the capture.
The program couldn't be much easier to use. Move your mouse cursor to the corner of the window, as usual; right-click when the cursor changes to the sizing icon, and select your target size from the list.
Sizer's menu opens with some standard sizes (640x480, 800x600, 1024x768, more), sizes grouped by aspect ratio (4:3, 16:9, 16:10) and YouTube dimensions (240p, 360p, 480p, 720p... up to 2160p).
This list is completely reconfigurable. Delete some or all entries, add others, do whatever you like until you've got a list that works for you.
Sizer's macro and expression support takes the package much further, at least in theory, enabling resizing and repositioning multiple windows with a single hotkey. This didn't even begin to work for us, though, and it's not properly documented yet either, so we'd recommend sticking with basic resizing for now.