Gimp

GIMP stands for Gnu Image Manipulating Program and that's exactly what it does.

Multiexposures

1. Make sure the layers window is open (CTRL+L)
2. Open your first image, it will be the background layer
3. Add New Layer
4. Open your second image, and paste it onto the first one, it will be the layer you just added
5. highlight this layer and choose "Add Layer Mask" and choose "White (full opacity)
6. You can now use your paint brush/eraser to merge the two images