Normal hosting facilities let you do pretty much anything you want, unless you start causing problems for the ISP or their customers. You pay them to provide bandwidth, space, power and cooling.
There are more restrictions for shared virtual sites (i.e. the $10/month web sites). Usually they just let you upload PHP and HTML pages and access a MySQL database. But, as Brandon pointed out, even they usually let you do basic things in PHP such as sending email.
- Laird Popkin, CTO, Pando Networks