Yes dude you can.
If you would like to host just one or two websites on your mac I would recommend putting your files in:
Hard Drive/Library/Web Server/Documents/
If you place the site here then you would have a url of
http://youripaddress.com/
If you decide to place the site in your home folder's area, for example
/luigomez/sites/ you would have a url of:
http://youripaddress/~username/
If you do this, you can host as many sites as there is users, but with long links.
Once you do that all you have to do is fix the port forwarding in your router to send the traffic to the mac that is hosting.
unless you have a great connection to the computer hosting the site, I would not recommend hosting it there. Cable connections usually have around 700Kb/s upload or less and if you have numerous people connecting it will sabotage your internet usage at home. You also have to leave the computer on or the site goes down. Personally I would not do this unless I was testing a website before I set it out to the wild, with cheap hosting rates from godaddy and others, I save more paying the $3 a month than keeping my computer on at all times wasting my internet, energy, and damaging my pc by leaving it on 24/7.
Hope this helps, cheers.
Edit. From what I can remember, Mac OS X comes with apache and you turn in on when you select personal web sharing. If you do decide to install PHP or MySQL you would have to download them and install them.
