Well, I don't know exactly what services you'd like to consume, but here's an example of a simple rest call:

public static void main(String[] args) {
        call("{'username' : 'simon' }", "http://www.example.com/user/");
    }

    private static void call(String req, String url) {
        URLConnection conn;
        String result = "";

        try {
            conn = new URL(url.toString()).openConnection();
            conn.setDoOutput(true);
            conn.setRequestProperty("Content-Type", "application/json");

            OutputStreamWriter outWriter = new OutputStreamWriter(conn.getOutputStream());
            outWriter.write(req.toString());
            outWriter.flush();

            String cookie = conn.getHeaderField("Set-Cookie");
            System.out.println("Cookie : " + cookie);

            BufferedReader reader = new BufferedReader(new InputStreamReader(conn.getInputStream()));
            String line;

            while ((line = reader.readLine()) != null) {
                result += line;
            }
            outWriter.close();
            reader.close();

            System.out.println("Received " + result);
        } catch (Exception e) {
            // TODO : Error handling
            System.out.println("Server Exception : " + e.getMessage());

        }
    }

Hope that helps :) Note that you still need to process the result using a JSON library, but that's pretty specific. If you need more help on that I'd recommend either reading up on the web on parsing JSON in java or asking a new question (although parsing JSON is a little offtopic for this forum, you should probably check stackoverflow for more info)

REST web services are not supported yet by the webservice call action within microflows.

You can of course use a Java action to execute a REST call, see the answer of Achiel for an example.

http://www.mendix.com/blog/consuming-first-rest-service/