I have a few custom gradients but the majority of the time I just take whatever random color I want and use the default gradient setting (where it goes from color to transparent) to lay it on the image in different spots.
If you search google or dA for some custom gradients a ton of neat ones will pop up. The set I have found and like is called "Tuscany Skies" [link]
Different colors have different effects depending on the layer mode too. Mine are usually set to "overlay" or "soft light" mode.
If you have any other questions I can make a tutorial for ya C: