In the population that frequents this discussion board, tinnitus is most likely a result of hearing damage from exposure to loud noise.
90% of the time, this is the primary cause.
However, it may also be a side effect from certain medications or substances, so this is something to rule out before you let the ringing drive you off the roof of a building.
Aspirin, other salicylates (I don't know if oil of wintergreen would cause tinnitus, but it's a salicylate, so I'll mention it here)
Other nonsteroidal anti-inflammatory drugs
Loop diuretics (furosemide, others)
Benzodiazepines (valium, xanax, others)
Zoloft
Calcium channel blockers
Chloroquine
Fluoroquinolones (cipro, levaquin, avelox)
Tetracycline, erythromycin
PPIs, such as nexium, prilosec, protonix
There is such a thing as "objective tinnitus", which can be heard by the examiner as well as the patient. This can be caused by vascular abnormalities (carotid artery stenosis, AV shunt) or some other issues. The upside is that this type may be surgically correctable.
'zilla