Have a look into an .ipk file (used by the ipkg or opkg manager)

An .ipk file is basically just a .tar.gz file. So you can unpack it using the tar command:

tar xvfz some-package.ipk
# or with newer tar versions:
tar -xf some-package.ipk

Some .ipk files may also be of a different archive format so that you need the following command to unpack:

ar x some-awesome-package.ipk

In the end, one of the above commands will unpack the file into ./debian-binary, ./data.tar.gz and ./control.tar.gz.

(You can then unpack the ./control.tar.gz file like this:)

tar xvfz control.tar.gz