Membership sites are now recognised as a great way to develop your own community and keep customers coming back for more. They also enable you to make a visitor a member immediately, even if they buy nothing and become a free member. You also make them an affiliate with their own affilaite link straight away so you can immediately start viral marketing.
If you have a passion or know lots about a subject then membership sites are a great way to go. One of the best sites for teaching you all about setting up and running a membership site is www.MembershipMillionaire.com.
Alternatively, if you would like to integrate a membership site with a WordPress blog (great idea according to my principles of keeping is simple) then take a look at http://wpwishlist.com/ which is a WordPress plug-in. And if money is a problem then the most basic membership site can be created with a WordPress blog just using the built in features.