How to give the visitor sorting options, including "sort by most popular"?

Hello, I’m looking to make a Photoshop resources site. I’m wondering how to offer on the index and category pages (categories would be: brushes, tutorials, etc) a drop-down menu with sorting options – probably just “newest” and “most popular”.

If popularity is too hard to calculate (for example the best measure of popularity on a resource like brushes would be how many times the set was downloaded, but number of “downloads” may not be an applicable figure for something like tutorials) I guess I should consider sorting by “most viewed” instead?

Might someone suggest a download counter, however? For resources that would be downloaded.

Sorry for the lengthy question. Any points in the right direction would be appreciated!