You can query all the packages installed on your system by including the -a option. If you pipe the output through sort and into a pager, then it's a nice way to get a glimpse of what's installed on your system. For example:

# rpm -qa | sort | less
[output omitted]

Here's how many rpms we have installed on one of our systems:

# rpm -qa | wc -l
    287

And here's how many files are in all those rpms:

# rpm -qal | wc -l
  45706

Here's a quick tip: Using rpm -qa can ease the administration of multiple systems. If you redirect the sorted output to a file on one machine, then do the same on the other machine, you can use the diff program to see the differences.