Cross platform, freeware life poster using XnView
If you have seen the instructions on how to create a life poster on the Mac using iPhoto or on the PC using Photoshop and wondered how to do this using freeware, then

Step 1:
Download Xnview for your platform.
Step 2:
Collect all the images you need into a directory. I am using 30 random images from a recent trek each of size 3008x2000 as an example.
Step 3:
Open XnView and browse to the directory containing the images.
Optional : Resize the images using "Tools->Sequence Convert" to the size/ratio you want or crop the images. A 20" by 30" print may require a 3:2 ratio. This can be done later also on the final image(which is what I did).
Step 4:
Select all the images and click on the "Tools->Create Contact Sheet". Time to do some math...and select the parameters you need.
This is the setup I used:

I have divided the 30 images into 6 columns and 5 rows. Considering my individual image size as 3000x2000, this results in a combined image of approximately 18000x10000. I have chosen half that(one fourth if you think about it) and hence set the final image size to 9000x5000. Filenames, headers, footers are all your choice but I have not selected any of those options.
Click on the "Create" button and wait for XnView to do the magic. With some luck on the parameters, you should get an image properly aligned.
Step 5:
Optional : Resize the image to your desired ratio(eg 3:2).
Save the resulting image in the TIFF format and you are ready to go.
My sample TIFF was 131 MB in size: