If you rooted your gadget or installed a custom ROM, then you have more freedom to install apps from wherever you want. SlideMe Application Manager is one of the biggest third-party marketplaces that helps you find apps, but it's not one of the best.
In order to install SlideMe Application Manager or most of the apps it offers, you will need to know about .apk files and other alternate download options. Once you do install it, most of the results you get are apps that exist in the Google Play market anyway. This is fine if you're not using an official version of Android, but limits this app's usefulness if you are. You won't find many powerful utilities on the marketplace. It's mostly joke apps or games that lose their novelty after some time.
The main reason people root their phone is to have more choice and control over their phone. SlideMe Application Manager takes away most of that choice by offering up a small sampling of boring apps you could already find in Google Play. Stick with that if you're running a stock Android build. If you're running a custom ROM, use one of the other available app marketplaces or just find apps online.