I have noticed on these forums that there are several posts that answer questions with arguments that can be passed to script files. For example, I've seen now that you can use price_group and hide_paysys with signup.php. I've searched on the wiki and on the forums and I can't find a complete list of options like this, does one exist? Or, perhaps you all could spew out the ones that you know that are the most 'useful'? (perhaps the two I listed are the only ones!)
check signup.html for all input fields listed, and you'll get your answer... as you can pass any variable that has an input field there. and yes, if you've got time to write them all out and list them here for others, feel free to as well
In addition to price_group, other big ones for me are: product_id: for auto selecting products by default. You can specify multiple using product_id[]=1&product_id[]=2 paysys_id: for auto selecting a payment provider: