Finding connected users

Hi,

I posted this question before on this list but got no replies and
still didn't find the answer myself, so here's another attempt:

How can I find the currently connected users? I.e. what query must I
use from a TCP/IP client?

Thanks,

Marc

----------------
Original question:

I would like to know what the best queries are to get system status info like:

Number of connected users
List of connected user names with the database to which they are connected
List of used databases with number of currently connected users
Last startup date and time of PostgreSQL
IP address

etc. (If anybody can think of anything else)
I already know SELECT VERSION() ;-)

(on version 7.3, on a client over TCP/IP)