There is a Java API which is linked below:
http://www.microsoft.com/en-us/download/details.aspx?id=22616
Other than that a few google searches resulted in the following discoveries: http://msdn.microsoft.com/en-us/library/bb130146.aspx
The link below tells you how you can create new work items via a URL: http://msdn.microsoft.com/en-us/library/bb822136%28v=VS.90%29.aspx
** As per jessehouwing's comment below *
This only works on 2008 and 2010 I suspect. There migth be a similar Url for 2012 and 2013, but the whole WebAccess feature was rebuilt using ASP.NET MVC instead of WebForms, so the wi.aspx no longer exists. I'm not aware of an alternative.