Valid casts & datatypes?

I've been going through the PostgreSQL documentation. I have not been able
to find what "casts" are valid. I.e. I can cast an int to a float by
putting a ::int4 on the variable name. I cannot cast a datetime to an
int. Is there a list of valid casts (from datatype -> to datatype)?

Also, it would be nice to have a short explanation of what the valid
datatypes are. In particular, I've run across some that I can't really
figure out (e.g. tinterval). It would be nice to have a single list of all
PostgreSQL datatypes and their ranges and general usage.

I know, I know, this is an open source project, so I could try to read the
source code and then share what I've found out. But I'm hoping that
somebody has already done this, because I'm not all that good at reading C
programs. Especially really, really complicated ones like PostgreSQL.

John