Engadget has a HOWTO for creating your own keyless entry system. For under a hundred bucks, you can build your own system and have doors buzzing in no time.
As the commenters on Engadget point out, this project seems particularly suited for apartment buildings with existing buzzers. Even then, it is not without issue. Any phone call to the line connected to the system will open the door. That could probably be remedied with some more technology to check caller ID or ask for a security code.
If you attempt this project, you might want to keep it on the sly. If the building catches wind of what your are doing with their intercom system, they are probably going to be a little upset.