Jason Conger is quickly becoming one of my favorite Citrix-focused coders. He's just released version 1.2 of his "Web Interface for Resource Manager." This is a full-fledged ASP.NET 2.0 web application that you install on a web server and point to your Resource Manager Summary Database. It then provides an interactive website where you (or your boss) can view usage reports about your Citrix Presentation Server 3 or 4 environment.
There are a few other products that do this now. XTS has a product called Access Tracking Manager that seems similar. Also, Citrix has a free Resource Manager Semantec Model which a snap-in for SQL Reporting Services that lets you run reports against the RM Summary Database.
However, in terms of free solutions that can be up in running in just a few minutes, Jason's application seems to be the way to go.
Rather then posting a bunch of screenshots here, jump over to Jason's site to check out screenshots of the latest version of this product. Needless to say it's pretty full featured and lets you see graphs on utilization (with filtering by app, users, servers), session details, client platform reports, etc.