It's built in.
In a terminal (PC / More Applications / Terminals) select either konsole or xterm.
Navigate to the place you saved the iso.
Type the command, "md5sum" (without the quotes) followed by the exact name with extension of the iso.
In a few moments, it will respond with the string which is the md5sum you can compare with the one from the download site.
Also, our burning software, k3b, does also compute the md5sum before you burn an iso.