As digital photographers, we have the luxury of having complete control of the digital darkroom. We have not always had this, but with great power comes great responsibility. (with apologies to Stan Lee and John Romita Jr.) Your responsibility? Make sure that BEFORE you start working in the digital darkroom, make sure your tools are not lying to you.
Here's a secret. Unless the magical unicorns of editing sit on your shoulder, your monitor is wrong. If you are still using a CRT, don't spend money on a calibrator, go buy a decent IPS display and come back after you've done that.
IPS or In Plane Switching displays are a great place to start with for a display. But maybe you have a general LCD, or LED powered LCD or a laptop display. Then by default you are seeing issues. Most all monitors these days have the ability to change their brightness automatically. Good for word processing and browsing, bad for editing. Most all monitors come set from the factory to eyeball stunning brightness, crushed gamma and high contrast. Why? So they look "better" on the wall in the retail store. It's just like a TV. If you bought your TV at a big box store, it was on display in "demo" or "vivid" or "retina burn" mode. You get it home, turn it on and it hurts. If you play games on your computer, many games "reconfigure" your display for brightness and contrast, or to make the dark scenery legible so the monster doesn't eat your face, you turn the display way up.
Enough kvetching on my part. A display calibrator reads the display, measures the display capabilities and produces a display profile called a display ICC. These are just like a printer / paper ICC file but for your display. There are lots of vendors out there doing this with the best known being the Datacolor Spyder, the Huey and the X-Rite Colormunki.
I have owned all three. The Huey was incredibly inconsistent. Software updates were rare and doing multiple monitors cost extra. Bad - do not buy. Until the release of the Spyder 4 Elite (and only that model), the calibrations were never consistent and Datacolor's software is still serialized to a single computer. Bad choice - do not buy. The Colormunki line consisting of the Photo, the iDisplay, the Display and now the Smile are easy to use, have great software, are fast and consistent. The Smile is new and sells for about $120. If you don't need to profile printers or paper or projectors, buy the Smile, keep the rest of your money in your pocket and get to the calibrating.
I've spoken about calibration on the TV show and at meetings of the Newmarket Camera Club. A very seasoned member, who has developed a healthy cynicism about tech sent me this email the afternoon after he bought a Smile.