yes object oriented programming is definately applicable to web applications like a message or bulletin board. another avenue i would suggest focusing some time on, if you haven't already, is data modeling. oo design and data design almost go hand-in-hand, and no place better than the web. doing so will help to identify proper placement of object properties.

in most instances a class can be a direct reflection of the storage layout for the data. in other instances it makes more sense to divide the data in different ways physically (to maintain a specific level of normalcy) and logically present it in the way envisioned.

as far as some projects to get you started; a bbs is an excellent one. the possibilities are really endless: web-based email, messaging, host/network/service management, job search, project management, billing and accounting systems, shopping carts (there's just something repulsive about that phrase)...calendaring, group-calendaring,...,....,....

but ultimately i would built something that you will want to use on your own and share with others. -or- that you can sell and bring in the dough with.