I use Bootcamp to install windows 7 on separate partition (200 GB for OSX & 50 GB for Windows) Now I decide to delete my windows and want my 50 GB partition back and stick to the 200 GB partition, How can I do this?
Using the tool Disk Utility, select your hard disk, then select the "partition" tab that appears on the right. This will allow you to list the partitions on your drive. If you used a standard bootcamp install, you will probably find that the windows partition appears after the end of the Mac one, and is at the end of the disk.
Selecting this partition, and hitting the minus button should remove it, and will leave a blank hole in it's space. Now you should be able to select the first partition, and increase it's size in order to fill the available space vacated by the windows partition.
Top tip: Backup your Mac partition! Use Time Machine, Super Duper, Carbon Copy Cloner of similar tools. This should be perfectly achievable without even a reboot, but you are mucking around with your disks partition table, risks do exist, there is always a chance of failure ruining both partitions. I wouldn't worry it about it too much, but if your data is important, you should do this first.