Bored with your current flavor Android, but don't want to flash a new ROM? You're in luck. Firefox has today released an interesting "Developer Preview" version of their web-based Firefox OS.
The Firefox OS 2.5 Developer Preview is an experimental app that lets you use an almost-complete version of Firefox OS your device — without having to flash anything. The 88 mb APK behaves like any launcher you would find on the Play Store. However, it also has several system apps for making phone calls, messaging, sending emails, an app store etc. Of course, you can still launch your regular apps.
It's a Developer Preview for a reason and the animations are rather slow on the device I was testing on (but it is much stable than you'd expect). There are some interesting UI ideas and features in Firefox OS, and since Firefox OS is catered towards lower end devices, it's unlikely most people have tried Firefox OS. It makes for an interesting exploration. Head on over to the site to download the APK. There's also an option to flash the OS if you want the full experience.