Windows: SnapPea is a free utility that lets you manage the apps, music, photos and SMS messages on your Android phone from your desktop. Think of the app as something of an "iTunes for Android", without the terrible performance.
If you've been looking for an all-in-one utility to manage your Android phone, SnapPea might be worth checking out. That all-in-one experience is clearly SnapPea's biggest draw: It offers one application that lets you manage everything about your phone. Connect your phone to your PC via USB (with USB debugging on -- it uses ADB for file transfers and backups) or pair it with the app over Wi-Fi.
From there you can manage, organise and transfer files to or from your Android device. You can install apps directly, bypassing Google Play if you choose, organise and update your phone's contacts, back up your phone, and move photos, music and videos. SnapPea allows you to send and receive SMS messages on your desktop through your Android phone.
SnapPea's clear competitor here is previously mentioned AirDroid, and there are pros and cons for each. AirDroid works using a web browser, meaning it's more cross-platform than SnapPea, but SnapPea lets you manage a few things that AirDroid doesn't. Still, which one works best for your needs is up to you.
SnapPea is free for Windows systems, and you can grab the installer below. There's no OS X or Linux version, but Mac users can download SnapPea's APK installer if you need help side-loading apps to your Android phone.