Even if you can't host multiple domains on a single plan, i would presume you could still create add-on domains that would be accessed by their TLD still. An add-on would just use resources for your original hosting plan.
Yes and no. Many web hosting providers only let you have addon domains which function like forwarded domains - which suck. Usually you can't even use masking with your addon domains and your browser will not show your domain properly. More details on domain forwarding:
http://www.multipledomainswebsitehosting.com/domainforwarding.htm
So, if you want to host more than one domain / website, make sure you find a host that enables you to do that properly without any hassle and problems of forwarding etc. There are plenty of really cheap hosting providers which let you do this...
Juuso