You can install apps on the HTC Aria using the ADB client.
You'll need to install the android SDK on your PC, and then use ADB to install the .apk files.
To install the android SDK, follow the steps in the Android Developer's guide to setting up your phone for development.
If you're on Windows, you'll need to go to HTC to get the device drivers, as Google only provides drivers for their devices. For some reason HTC doesn't publicise the ADB driver well, but it is included in the HTC Sync software. You can grab the latest version from HTC's Aria downloads page.
Mac and Linux users do not need to install device drivers to use ADB.
Finally, connect your device to your PC, making sure that debugging mode is enabled on the device. You should see an entry in the system tray stating that the device is in debug mode. On your PC, run the following command:
adb install yourapp.apk
This will install the app on your phone, and you'll be good to go! Note that you'll either need to have the android-sdk/platform-tools directory in your path, or run the command from that directory.