As for online sites, couldn't tell you. I just use them, mostly for mountain biking, to quickly get trail/gps data in .gpx format to create ride routes.

However, if you want to put mapping software on your PC, I have 3 free quality options for you that will all provide you with the function you seek. The 3 programs are Garmin MapSource, Topofusion and Google Earth. I use all three programs, depending on what I'm wanting to do. Here is how to install each program and get the UTM data displayed.

MapSource:
This one takes a few steps to install, but it's really not a PITA (Pain In The ***). MapSource normally comes with Garmin GPS units, but this is how I installed the program before I even bought my used GPS Map60csx. This is a pretty well known method for being able to get a functioning version of the software on your system.

First, download and install the Garmin Training Center software.

http://www8.garmin.com/software/TrainingCenter_351.exe

After installing TC, download and install the current MapSource software. The following link is subject to change as Garmin updates the software.

http://www8.garmin.com/software/MapSource_61511.exe

Now you will have the MapSource software installed with a simple US base map.

To get some topo data, go to GPSFileDepot and download and install the map(s) of interest to you.

http://www.gpsfiledepot.com/maps/

Now that you have installed some topo maps, start MapSource and click on the dropdown box below the menu bar (top left cornor of the window)and select a mapset that you installed.

As you move the cursor around the map, notice the Lat/Lon coords on the status bar at the bottom of the screen. To change this to display UTM coords, click on the Edit menu, Preferences... In the Preferences window, click on the Position tab. From the Grid dropdown menu, select UTM, then click the OK button. Now you should have the UTM coords displayed in the status bar. You're good to go from there.


Topofusion:
This will allow you to install the software in Demo mode, which will put translucent 'nag' graphics over top of your maps. Purchasing the software removes the nags, but the software is quite usable even in demo mode.


First download and install the Topofusion software.

http://www.topofusion.com/TopoFusion-Demo-Pro-3.97.exe

After installing the software, run it and go to the Map menu and select the type of map you want to display; you can do topos or various satellite or even combo maps. The map data/graphics are downloaded from online servers and then permanently stored locally to the hard drive.

Now, go to the Options menu, Preferences. In the Preferences window, click on the Units tab. Click on the Coordinates type dropdown and select UTM, then click the OK button. The coordinates will display in the right side of the button bar at the top of the screen, below the program menu bar. Done.

Google Earth:
Lots of fun features and great for creating routes just by picking out roads and trails from satellite images. However, you won't have the topo option available.

Download and install by going to the following link and agreeing to terms and such...

http://earth.google.com/intl/en/download-earth.html

After installing, run GE, click the Tools menu, Options... In the 3D View tab, go to the Show Lat/Long group option and Select Universal Transverse Mercator, then click the OK button. The coordinates will display in the status bar at the bottom of the window. And that's it.

Install them all, play around (they're all pretty easy to figure out) and enjoy!