We have added the ability to retrieve the "latest" value in several places in MOOSE. We use the adjective "latest" to indicate that these values come from the most recent file written rather than a solution file with the highest serial number. This situation might occur if you are rerunning the simulation in the same directory over and over without cleaning up in-between runs. In this case you may have an older simulation that ran longer than the most recent simulation. So "latest" means "most recent".
This blog post is here to summarize these capabilities
If you just want to grab the values from the "latest" timestep in a previous simulation and you don't know the specific timestep number.
[Variables] [./u] type = LAGRANGE order = FIRST initial_from_file_var = u initial_from_file_timestep = 'LATEST' [../] 
If you want to create a "restart" simulation using the "latest" solution from a previous simulation.
[Problem] restart_file_base = 'my_simulation_cp/LATEST' 
If you want to use SolutionUserObject to load a solution from the "latest" solution from a previous simulation.
[UserObjects] [./soln] type = SolutionUserObject mesh = my_mesh.e system_variables = vars timestep = 'LATEST' [../]