loss of anti-freeze problem

I have a 1998 GMC Sierra 1500 model with about 100,000 miles.
When I park the truck, a puddle of engine coolant puddles under the engine. This doesn't happen all the time, but sometimes leaves a big puddle. The heat doesn't work properely either.
Is the thermostat sticking, or is the water pump going, or is the radiator semi-clogged?

It could be a leaking radiator ,hose ,or the waterpump is gone. look under the water pump to see if the weeping hole is wet,if so change the waterpump.
Again,check to see where it's leaking.
If the heat isn't working well,the antifreeze is probably low. If the level is good,the thermostat could be sticking open or closed. Does your heat gauge show normal and no heat from the vents?

Kevin