On the other side of the spectrum are partial tears or complete tears of the tendon. This condition leads to more significant disability, including strength issues and difficulty walking - certainly symptoms that are more severe than a case of tendonitis.
If the symptoms of soreness and stiffness in the Achilles region are mild, then you can try some simple things to quiet the problem down:
First, gentle stretching (the runner's stretch first thing in the morning) can help. No hard or quick stretches (bouncing), but just a slow controlled stretch for a few minutes for both heels can be beneficial.
Secondly, simple over-the-counter anti-inflammatories can help with the inflammation and pain. Take as directed for a short period of time. Some people shouldn't take anti-inflammatories if they have certain conditions or are on specific medications, so be sure to contact your PCP to see if these drugs are right for you.
Third, if the tendon is really inflamed and sore after a workout or long day at work, try an ice massage to the area with a few ice cubes over 10-15 minutes - sometimes this can do wonders.
Finally, trying a small (½ inch or less) heel wedge inside your shoes has two advantages - one, it makes you taller! But more importantly, it can relieve some of the stress on the tendon and can alleviate some of the common symptoms.
These simple treatments are truly for a mild to moderate case of tendonitis. If the symptoms have been around for over six weeks, then a visit to your doctor is probably needed to see if further evaluation with an MRI is warranted.