Would look nicer if the tabs were a different color (less blue etc.) I think it's a bit plain personally but I like the index/ register. 8/10 good job.
Good luck with this however it's gonna be pretty difficult to make it unique as we already have sites such as facebook, twitter etc. so that's something to take into consideration.
I like number 3 the best, also for the online users bit in 4 try making the box a bit smaller and rounded. For your first banners these are real good though.