Ethnoancestry, a new company is offering sub-haplogroup determination tests. This may be interesting to those who have already determined their main haplogroup and may be interested in seeing whether or not they have a more derived lineage. One should be careful when ordering a test of this type, since the information that can be obtained varies in different haplogroups.
Let's take haplogroup J, for example. Someone who belongs to it can learn that he is either J1 or J2 with an extremely small probability that he will turn out to be J*(xJ1, J2). On the other hand, some haplogroups such as R1a don't have a strong substructure (yet) as defined by known SNPs, so it is not entirely clear whether one should wait for more research to be published on the matter, i.e., to figure out which SNPs are interesting, widely prevalent, and worth testing for.