Split Toning

I found an article on DPS (http://www.digital-photography-school.com/create-amazing-sunsets-with-lightrooms-split-toning-tool) that explains what the split toning tool in Lightroom does and some color suggestions to really make sunsets pop.  After playing around with some other images, I found that this tool also worked well for particularly cloudy pictures.

These are both of the Memorial Tower on LSU's campus.  Originally the background was almost all orangey yellow, but split toning gives it a very cool and interesting effect that makes the photo more interesting.

Just a note, and the article above mentions it - split toning will affect the entire image, so it is important for silhouettes that the foreground is truly black - that way you don't have to take extra steps after split toning to fix the foreground.