If you can't wait for the over-the-air update that will give Android 2.2 (Froyo) to the HTC EVO 4G, then you'll be delighted to know that the update file can be downloaded from HTC [update: HTC pulled the file; see comments for alternative links], which means you can apply it manually. Best of all, you don't even need a computer to do it.
Note: HTC says the file is not final and may cause problems; however, everyone who has applied it so far (including me) has encountered no major issues. Your mileage may vary, though, so please continue at your own risk.
Here's how you can apply the update using nothing but your EVO.
1. If you don't already have it, download ASTRO File Manager from Android Market.
2. Download the update file (see comments for alternative download links).
3. Tap on it and open it with ASTRO.
4. Hit the Back button on your EVO and long press the update file name.
5. Tap Edit -> Rename to rename the file to "update.zip" without the quotes. Press OK.
6. Long press the file name again, tap Edit -> Move, hit the Back button (you should be in /sdcard now), and press Menu -> Edit -> Paste.
7. Turn the EVO off.
8. Hold the volume button down while powering the phone back on. From the white bootloader screen, select "Recovery" from the list. Use the volume button to highlight it and the power button to select it.
9. Press the volume up and power buttons when you see the triangle with an exclamation mark in it.
10. Select "apply sdcard:update.zip" from the menu, again using the volume button to highlight and the power button to select.
11. When the installation is complete (be patient), you'll see "Reboot via menu to complete installation." Use the volume key to navigate to "reboot system now" from the menu at the top of the screen and the power button to select it.
12. Don't panic. This will take some time and the EVO will restart a few times.
Thanks, Chris & OneWing!