A map of the Olmec heartland.  The yellow sites are known Olmec villages and towns.  The smaller red dots mark locations where artifacts or art have been found unassociated with habitation.  The river courses and shorelines are modern, as are the names (since we do not know the names the Olmecs used).
The Olmec heartland is the southern portion of Mexico's Gulf Coast region between the Tuxtla mountains and the Olmec archaeological site of La Venta, extending roughly 80 km (50 mi) inland from the Gulf of Mexico coastline at its deepest.  It is today, as it was during the height of the Olmec civilization, a tropical lowland forest environment, crossed by meandering rivers.
The Olmec heartland was the home of the Olmec culture, which became widespread over Mesoamerica from about 1400 BCE until roughly 400 BCE.  
The major heartland sites are:

San Lorenzo Tenochtitlán
La Venta
Tres Zapotes
Laguna de los Cerros - the least researched and least important of the major sites.Smaller sites include:

El Manatí, an Olmec sacrificial bog.
El Azuzul, on the southern edge of the San Lorenzo area.
San Andrés, near La Venta.Important heartland finds not associated with any archaeological site include:

"The Wrestler", a basalt statue found at Arroyo Sonso.
Las Limas Monument 1, found by two children looking for somewhere to crack nuts.
San Martín Pajapan Monument 1, found high on the slopes of San Martin Pajapan.Note: This is another version of Image:Olmec Heartland Overview v2.svg
Olmec Heartland Overview 4.svg
Olmec Heartland Overview 4.svg: Madman2001
derivative work: Rowanwindwhistler (talk)