I work in my company's HR Department, and I've typically spent most of my time recruiting.  We aren't exactly in a hiring freeze, but requisitions are way down.  I'm trying to pick up the slack by helping with administrative tasks, but I want to make sure there are no doubts that I'm still a valuable team member.  What else can I do to help my managers and peers?

Odds are that the company still sees you as a valuable employee.  They might even foresee hiring picking up in the not-too-distant future, but with your newly found down time, try doing some house keeping. 

Get to all the smaller projects that have been lingering on your "to do" list, and when you finish with those take a broader look at the department.  Are there processes and procedures that should be updated or streamlined?  Are there recurring functions that you perform that currently have no defined global process?  Try putting all of these in place so that when things do pick back up again, you can do your job faster and more efficiently than ever before.