3 lights 2 have gels 

Blue gel from behind.

along with a ringflash.

Red gel on gray paper.

I like them to emphasize the colors on a sunset (pick the gel opposite the color you want to enhance and set a custom white balance on the gelled flash and the ambient gets a nice push the other way). 
They're also great for changing the color of a white wall or similar background. 
Occasionally I'll use them on a fill light to give a shot a different mood. 
Less important in an era of easily changed white balance, but CTO gels (or work up your own) are good for mixed light sources (CTO for flashes in tungsten environments)

I like to use them as an edge light, this one has a blue gel on a strobe to the left of Jade.  You can see the outline in blue on her left side.

They are inexpensive and fun to work with.

Enjoy!

Photeus

This was a subtle use of gels on a hotshoe strobe.
The strong sunlight coming through the window wasn't direct but was being bounced off of a bright red brick building across the street. The light had a very appealing warm quality. I wanted to emphasize the contrast in the cooler ambient light in the room with the red-brick warmth of the window light.

I gelled a single strobe with a cool CTB gel to give a slightly bluish quality to the shaded areas of the room. I got my subtle split tone effect in camera.

18+ photo with partial nudity
http://www.modelmayhem.com/portfolio/pic/30928816

Bored one day and just playing in the studio... yellow gel left of camera, blue gel right of camera, with barndoors and a honeycomb grid to cut down on spill.