Residency spots open up all the time. People drop out. People move. People switch programs. If you didn't match into a residency this year, you might still have a chance at post-graduate training by jumping into a residency during the fall or spring.
If you're looking for an open residency spot, here are some helpful resources:
http://www.openspots.com (website might be down)
You can also find discussions about open spots on these forums:
Don't forget these useful residency links:
http://www.NRMP.org -- National Residency Matching Program website
http://www.ama-assn.org/ama/pub/category/2997.html FREIDA (Fellowship and Residency Electronic Interactive Database)