The "+" indicates that there is an ACL (Access Control List) entry
associated with the file. I don't know about Linux, but ACLs are used
on Solaris too. See the manual pages for "getfacl" and "setfacl".
For files that have a default ACL or an access ACL that contains more than the three required ACL entries, the ls(1) utility in the long form produced by ls -l displays a plus sign ( + after the permission string.