How to write a simple webserver in Erlang?

Taking "produce" literally, here is a pretty small one. It doesn't even read the request (but does fork on every request, so it's not as minimal possible).

-module(hello).
-export([start/1]).

start(Port) ->
    spawn(fun () -> {ok, Sock} = gen_tcp:listen(Port, [{active, false}]), 
                    loop(Sock) end).

loop(Sock) ->
    {ok, Conn} = gen_tcp:accept(Sock),
    Handler = spawn(fun () -> handle(Conn) end),
    gen_tcp:controlling_process(Conn, Handler),
    loop(Sock).

handle(Conn) ->
    gen_tcp:send(Conn, response("Hello World")),
    gen_tcp:close(Conn).

response(Str) ->
    B = iolist_to_binary(Str),
    iolist_to_binary(
      io_lib:fwrite(
         "HTTP/1.0 200 OK\nContent-Type: text/html\nContent-Length: ~p\n\n~s",
         [size(B), B])).

For a web server using only the built in libraries check out inets http_server. When in need of some more power but still with simplicity you should check out the mochiweb library. You can google for loads of example code.

Do you actually want to write a web server in Erlang, or do you want an Erlang web server so that you can create dynamic web content using Erlang?

If the latter, try YAWS. If the former, have a look at the YAWS source code for inspiration

Another way, similar to the gen_tcp example above but with less code and already offered as a suggestion, is using the inets library.

%%%
%%% A simple "Hello, world" server in the Erlang.
%%%

-module(hello_erlang).
-export([
  main/1,
  run_server/0,
  start/0
]).

main(_) ->
  start(),
  receive
    stop -> ok
  end.

run_server() ->
  ok = inets:start(),
  {ok, _} = inets:start(httpd, [
    {port, 0},
    {server_name, "hello_erlang"},
    {server_root, "/tmp"},
    {document_root, "/tmp"},
    {bind_address, "localhost"}
  ]).

start() -> run_server().                                                       

Keep in mind, this exposes your /tmp directory.

To run, simply:

$ escript ./hello_erlang.erl

For a very easy to use webserver for building restful apps or such check out the gen_webserver behaviour: http://github.com/martinjlogan/gen_web_server.