I wanted to wirelessly share content from my Android phone to XBMC on my modded original Xbox as well as my laptop so I could easily show pictures or videos I took on my phone on a bigger screen without needing to copy the files over. There are web servers for Android that let you download files off your phone, and you could of course use something like Dropbox to get files onto your laptop, but I wanted something that lets you stream content easily to XBMC so I wouldn't have to wait for a download to complete. The Samba Filesharing app by funkyFresh lets you run a Samba/Windows fileshare server on your rooted Android phone, and this does exactly what I wanted.
The Samba Filesharing app is free and shares the contents of your SD card out over WiFi to anything on the local network that can access a Windows file share. You can set a username and password to keep things locked down. You can also specify the workgroup.
I had to add the fileshare to XBMC manually using the IP address the app listed in order to be able to see the phone contents. Browsing through XBMC's list of SMB file servers showed the phone but I couldn't connect that way for whatever reason. Not a big deal.
There are UPnP file servers available for Android that also work with XBMC and have the benefit of letting you stream content to things like an Xbox 360 or other UPnP compatible media player, but most of these are paid apps. I'm looking into free UPnP servers for Android, and will post later if I find a good one. If you know of one leave a comment and let me know.