The term you are looking for is diptych or triptych. If you search on those terms you will find what you are looking for. If you use photoshop or GIMP, you can use actions or templates to place multiple images and create borders.

If you want standalone program to do just this one thing, here is one free (open source) program that does a good job. It is called DipStych.

Download from here

Diptych is very easy to use. You browse for your photos, preview them, and can then set the size and color of borders. It will resize the individual images to be the same height and/or width

Here is an example I've done:

It will stitch images vertically or horizontally, but not both, but it is easy to use. You could stitch 3 images horizontally and create one image. Then repeat with your second row of three. Then pull those two images back into the program and this time stitch them horizontally. So if I do another:

And now stitch them vertically on top of one another:

You would need to experiment with the borders in each step since the last step has caused that middle border to double up.