I use org-mode for time tracking and can calculate the overall time with C-c C-x d. Is there a good way to show total time spent over several org-mode files (I tend to have separate projects in separate files)?
You can create a custom agenda command to assemble arbitrary tasks from various files.
You then have a couple of ways of showing the clocking information:
Along with the Agenda's clockreport, you can drop inline clocktables in to your Org files, which is nice if you're looking for something more static than what an Agenda will generate.
As @Ryan says, what you want is a clock table. The key is that the clock table can include several files. For example:
#+BEGIN: clocktable :maxlevel 4 :scope ("todo.org""journal.org") :block today #+END:
If you C-c C-c on that line, it'll generate a clocktable that sums time spent across the files "todo.org" and "journal.org". Use :maxlevel to specify how detailed you want the output to be (summarized at the top level of each file? summaries for each headline? Summaries for each headline at depth 2? Etc.). But you'll get a total summed across all files at the top regardless.