The PostgreSQL source distribution includes several examples of index methods implemented using GiST. The core system currently provides R-Tree equivalent functionality for some of the built-in geometric data types (see src/backend/access/gist/gistproc.c). The following contrib modules also contain GiST operator classes:
B-Tree equivalent functionality for several data types
Indexing for multidimensional cubes
RD-Tree for one-dimensional array of int4 values
Indexing for tree-like structures
Text similarity using trigram matching
Indexing for "float ranges"
Full text indexing