Most community based sites require a login so they can distinguish users from guests, etc. What would you say makes a good login page though? It is fairly reasonable to say that simplicity should be the most important factor to play in this - who would want to return to a site that is complicated to login to.
In terms of design it should be made to fit in with the site; but should it be hidden from view? Should it be a separate page? Is putting it in a lightbox acceptable? There are so many questions you should be asking yourself when you design a login form for your page, and it should be answered based on the overall design of your site, the requirements for the form, and the target audience. This very site uses a simple link at the top of the page that isn't hidden, and isn't overpowering either. Clicking it will bring up a lightbox dialog for users that can use CSS / JavaScript solutions. For those that can't it elegantly falls back to use a form on a standalone page. This means two forms that need designing separately with consideration for the surroundings.
Whilst you ponder on what the best way is to design a login form, a user on Flickr has compiled a great collection of login form design patterns.
