You can unlock HF access control systems that are looking for the UID using the Spark
An Arduino connected to an R522 or PN532 module would read the UID just like a normal tag. Some off-the-shelf door locks will work, but others might not play nice with the Spark. Here is a list of compatible products
Right now nothing compatible with the Spark is currently listed, but if you ask I’m sure the community will help out.