Module development and code questions

Is there a module that would allow one to sell downloadable content?

Hi everyone,

I'm currently redesigning a website with drupal (my first time using the system and I must say it is a slick cms). However, this is the first problem I have run into. I've created a custom calendar application module without any problems, as well as a few other small helper modules. All without problems... However, I've begun to create a new module to post news items dynamically to different pages.... problem is, the page isn't just printing out when I go to the menu path... it's printing on every single page, before the normal page that is supposed to be printed...

After looking at my site's statistics and views, I thought of putting the Suggestion box or something like that right after the post, so the reader can rate that comment/post/make a suggestion, like Y! uses: "Did this tip help you?" Is there a module for that? I've read on this before, but there wasn't really an answer on that...

Also, off this topic: how could I block my own IP from statistics, so when I check the hits, I don't see my own hits?

Also, is there a way to see the actual results in a graphical mode, like a math graph?

As a user authorizes with my site (www.mysite.com), in parallel and under-the-covers, I'd also like to authorize the same user with my Drupal installation (community.mysite.com).

I've created a CGI script which uses the libXMLRPC library from Dan Libby. It makes a HTTP POST call to http://www.mysite.com/xmlrpc.php script such as:

In this post http://drupal.org/node/29273#comment-129201
I used the script to get captcha working for the guestbook module. Well it works fine, the only problem I have is that if I open the guestbook, the error message "The answer you entered to the math problem is incorrect." without having entered anything yet or even clicked on the send button? What I want is that after clicking the send button, the error message just appears if somebody did it false or didn´t type in anything. Can someone help?

I am unable to find out definition of call_user_func_array($menu['callbacks'][$path]['callback'], $arguments);

Please help me where i can find def. of this function.