Opera has released a test build of their Opera Mobile web browser with support for extensions.
The labs build is available for Android platform. There are emulators available for desktop operating systems as well.
Opera Software spoke about this build:
This is a very early Labs release and it’s not quite certain at this point that extensions functionality will effectively make it into a future release of Opera Mobile; the design is also most definitely not final, and there are some rough edges here and there, but if that doesn’t scare you away – and why should it! –, you can find an Opera Mobile Labs build with extensions support for Android below. In addition, we’ve released extension-enabled Opera Mobile Emulator Labs builds, so you can test your extensions in a desktop-based Opera Mobile instance as well.