Flying VFR in uncontrolled airspace, what to do if one spots an apparently out-of-control fire on the ground?
This has happened several times and each time I was able to reach a local approach facility.  Prior to GPS, I would circle above the event, and approach would get a pretty good fix.  One time it was a propane facility near the uncontrolled airport I was flying out of at night.  Shortly there after, when the flames shot above 4500 feet, the tower saw it form 20 miles away, visually.  

One of the more interesting events was one early spring, when I was crossing the Finger Lakes in New York, and saw two snowmobiles go through the thin ice.  I hung around for the recovery, and everyone was pulled out of the water alive.  In that instance, (Rochester) approach kept asking for updates, as they were talking with the fire departments, who were trying to find the best ice for their approach to the snowmobilers.

In my experience, I have always been able to reach approach.  However if approach was not available, enroute or FSS, and as a last resort, local unicoms.   

When flying for the state conservation department, we did radio in fires, as part of the job expectation.  

For those looking for more guidance, might I suggest something like:

Cessna345: Approach, Cessna 12345, 20 miles south, near Geneseo VOR, with two snowmobiles through the ice on the lake, 5 miles southeast of the VOR.  Three souls observed in the water.

Apch: Cessna 345, squawk 4311, ident, say altitude.  How long ago did this happen.

Cessna345: About 2 minutes prior to my call up. 1500.

Apch: Any other snowmobilers in the area?

Cessna345: No, not at the north end of the lake.

Apch: Fire is being dispatched from both sides of the lake, would you say the swimmers are closer to the east or west shore.

Cessna345: East shore, advise fire that I am circling above the swimmers, with landing lights on.

and so on.  

Addendum: The question asked about uncontrolled airspace, and while there may be uncontrolled airspace even in areas of radar and ATC coverage, it is useful to discuss what to do when ATC coverage is more sparse.  The objective is to communicate to authorities so that fire services can be dispatched to control the fire, and alert people who might be in harms way.  My suggestion, as a CFI and as someone who has flown the northeast corridor as well as rather desolate portions of the earth, is to establish communication with anyone on the ground as soon as practical.  If you can reach any FSS or ATC facility, I would contact them first.  If unable, and if nearby (considering your altitude) I would try airports with unicom/multicom frequencies listed.  You might alter your flight to get you closer to civilization and potential contact points. In Northern Canada, as an example, there are many well equipped airports which are not charted, but support mining operations. I carried a list of their frequencies shared with me by regional pilots.  Barring that, I would make calls on 121.5.  There may not be any facilities around which are in range for 121.5, but there are likely airlines and military flights which monitor 121.5, particularly in cruise. It is not uncommon to use other aircraft to relay information, and short concise messages help.  If one has HF or a SATphone, then the radio game is different.  In extremely sparse areas, a fire may only be of advisory interest, so do not expose yourself to increased risk to report it.