Here's an online density altitude calculator.

Some typical numbers for my home airfield in the Summer:

Altitude (field elevation): 6840 feet
Air Temp: 90 degrees F (it has been hotter lately)
Altimeter setting: 29.92
Dew Point: 45 degrees F

Calculated Density Altitude: 10,416 feet

Here's another calculator, this one able to handle English or Metric units.

For some background on density altitude, go here.

If you look at the Koch chart on this page, you'll see that the density altitude example I provided above would result in a takeoff roll roughly 260% more than required at sea level and an 80% decrease in rate of climb compared to sea level. It also makes for some very long landing rolls. While the Indicated Air Speed (IAS) is the same, the true air speed (corrected for altitude effects) is much higher (about 11 knots faster than indicated).