psycopg is a PostgreSQL database adapter for the Python programming
language (just like pygresql and popy.) It was written from scratch
with the aim of being very small and fast, and stable as a rock.
The main advantages of psycopg are that it supports (well... *will*
support) the full Python DBAPI-2.0 and being thread safe at level
2.

psycopg is different from the other database adapter because it was
designed for heavily multi-threaded applications that create and
destroy lots of cursors and make a conspicuous number of concurrent
INSERTs or UPDATEs.  Every open Python connection keeps a pool of
real (UNIX or TCP/IP) connections to the database. Every time a new
cursor is created, a new connection does not need to be opened;
instead one of the unused connections from the pool is used. That
makes psycopg very fast in typical client-server applications that
create a servicing thread every time a client request arrives.

psycopg is still in development but that does not mean that it is
unstable. It just does not support the Python DBAPI-2.0 completely
and is missing some of the more esoteric features we want it to
have.