Confining the possibilities to those large enough to make a really substantial difference to energy projections, I come out with two fossil fuel technologies:
Underground coal gassification, and the use of methane hydrates.
For coal gassification potential reserves are vast:
http://www.energybulletin.net/node/11901

This summer, students from Norwegian University of Science and Technology analyzed data from 600 wells drilled on the Norwegian Shelf of the North Sea. They calculated that there are 3000 billion tons of coal off the Norwegian coast. Most of the reserves are located at Haltenbanken. This compares to today's proven and recoverable world reserves of 900 billion tons of coal.

Some feel that they could be developed in fairly short order:
http://news.bbc.co.uk/1/hi/england/7046981.stm

"I think this is a wonderful springboard to a renewable future. Now we have cleaner ways of using fossil fuels to get there.
"By drilling boreholes, no-one has to go underground. It's a much cleaner and safer opportunity for the people involved.
"In the North East we're not scared of novel industries. There's still an appetite for engineering in the North East. I think it will be very popular in the region."
He said he would like to see the process up-and-running in the region within five years.

As for methane hydrates, on land at least it seems that they may be able to be exploited:
http://nnsl.com/northern-news-services/stories/papers/mar31_08ma.html

Scott Dallimore, with Natural Resources Canada and chief Canadian scientist for the program, said the results of the Mallik tests were promising.

"A sustained flow (of methane) was observed," he said.

Since this is only a research project, and not a commercial venture, most of the methane was brought to the surface and burned off. There was no attempt to capture the methane.

The Japanese are interested in tapping undersea gas hydrate reserves off their coast, but they must first find out if it is economically feasible. The Mallik site is of interest to them because it is easier and cheaper to research the gas hydrates from the surface.

Possible impacts on the climate, of course, are another issue, but very large resources seem to be available.