As long as you are downloading just presets -- as opposed to plug-ins -- there is no risk of viruses since presets are just XML text files by another name.
Luxe Lens seems like a good place to see what is availble. I have never used any of their presets.
Most folks have opinions about which ones they like. I personally use only the presets I have created even though I have download a few collections of free presets from various places.
Anything in particular you are looking for in terms of effects or subject matter?