This is a related product module that can be displayed in the side column (or content top and bottom if you wanted). Updated so only shows the box if there are any related products.
It shows the same related products that appear in the tabs on the product info page but makes them more obvious by allowing them to be displayed in the side column. It can also be limited to x number of products.
In response to: Undefined index: product_id in /catalog/controller/module/related.php on line 19
I have uploaded another version (v2) which I have added some code in to supress the error. This I suspect will only mask the probem (and no related products will show - I don't know because I can't replicate the problem so i can't test it) but feel free to give it a go if you get that error. Be nice is someone can leave some feedback as to the results of this....