Spilt-toning harks back to the days of mono hand-printing and the darkroom. Chemical toners are used to add hints of colour in the shadows and highlights of the image - typically a warm tone for the highlights and cooler tones in the shadows. It's a technique that will add a lovely fine-art look to your shots, full of rich and sumptuous tones, and the good news is, it's possible to recreate this effect in Photoshop. To do this, you'll need to create a Gradient Map, and while this may sound daunting, it's actually a lot simpler than it sounds. The other thing to remember is that you're not restricted to specific colours for the highlights or shadows - feel free to experiment with your own colour/tone combinations to get your own distinct look. Let's see how we create a split-toned effect in Photoshop...
Select Gradient Map
Once you've converted your image to mono, bring up your Layers palette (F7) and at the bottom, click on the Create new fill or adjustment layer icon at the bottom of the palette. From the drop-down menu, select Gradient Map.
Make a Color Stop
Click on the Gradient to bring up the Gradient Editor dialogue box. From the Presets, select Black, White. Now click a quarter of the way along the gradient so the Location is 25%. Click on the Color, and pick a dark colour from the Color Picker and hit OK.
Now click three-quarters along to add another Color Stop - Location should be about 75%. Click on the Color again and select a light tone for the highlights and hit OK. Now hit New to save the Gradient in the Presets so you can use the gradient on another image.
The effect will probably look too strong, so making sure you have the Gradient Map layer selected; we can use the Opacity control to vary the intensity of it. Save as either a TIFF or Photoshop file to maintain Layers so you can always edit it at a later date.