I spent most of Sunday browsing for Lightroom add-ins. The most common are Develop module presets. I think having a good collection of these helps to quickly see what changes in mood and other possibilities might be for given photo. Starting with a straight shot of "Hercules and the Hydra" behind the St. Louis Art Museum, taken with a Nikon D100, 24-85mm @ 48mm, ISO 200, f/11 @ 1/400s, cropped 1:1 in Lightroom 2.
Transform using "BoonDoggle 2" from BlixPhoto. This is a very nice muting of tone combined with a strong boost in contrast. I like the way it grayed the sky, moving the shot to more abstraction.
Look familiar? This is the "300" transform by Mike Lao. This subject, oddly enough, isn't really suitable, but I'd love to run this on some protraits.
Probably not familiar because almost nobody saw the movie, but this is the "Max Payne Red" transform from by Matt Koslowski, the "Lightroom Killer Tips" guy. Not great for this subject, but I'm keeping handy...
Finally, this one really surprised me. The statue transformed from bronze to chrome, while leaving the other colors roughly the same. This is the work of the "Gritty & Grungy" preset from Neil Cowley.
Shoot me a note if you want more information about these Lightroom presets.