Beside using Windows Phone SDK to test the windows phone apps - xap files (those without source code), are there other ways (or alternatives or emulator) that I can use to test the xap files?
take a look at this forum might help How to transfer .xap file to Windows phone ?
Extract from the link given above:
That explains how to use WindowBreak to get free interop then developer unlock.
You'll need to then make sure Zune on your PC is up to date, and install a Microsoft .net framework - you'll want to Google how to do this - your best bet is to check the XDA Developer forums.
Once you have WindowBreak, Zune, and the Framework, the next step is to install a program that can sideload an .xap to your phone. There are quite a few out there - the one I use is wXAPassistant by WINCO from the XDA forums. Do some research to find out which one works for you.
Once you've done enough reading, and you have to do the reading in order to understand everything fully, you'll be able to sideload any app you want, install custom roms, and update your phone to the latest WP7 builds.
If you end up going the custom ROM route, I'm using an excellent Tango ROM that I got over from XDA. You can install .xap from browser and there are a lot of little customization options included and it's very stable. This is for Focus v1.3 but i'm pretty sure he released a v1.4 also so make sure and know what hardware version your Focus is.
Using a custom ROM after using WindowBreak to transfer xap files to Windows phone.
There's also a new Company Hub feature, which will allow companies their own app distribution pipe, as well as giving IT administrators the ability to highlight specific things depending on what's important to their organization.
Quote from engadget
So one way would be to create a a company account and distribute the xap file that way. You can download the required app at Microsoft. There are some prerequisites like System Center, so it is not feasible for an individual.