File Transfer on LAN using wifi
There are 2 options:


Root your phone.
Install the Samba Filesharing app, or any other Samba server for Android.
Install samba on Ubuntu, or any other Samba client (there are some on Android too). This should do: sudo apt-get install samba, or any other equivalent command on another OS. You don't need this step on a Windows machine.
Optional: Install an app on your PC to make it easier to configure samba on your PC. apt-cache search samba should give you some good results (or anything equivalent).


The point is that this Android application will let your device show in the "My Network Places" of Win XP, and the "Network" of Win Vista/7 and Ubuntu. From there you can browse to smb://ANDROID/ (or \\ANDROID for Win) in your file manager.

Make sure you set the WORKGROUP correctly on the Android app. And if you face some problems with accessing your phone, try using the IP address instead of the name (ANDROID by default).

Otherwise, if you're not rooted for example, you could install a file manager app with support for Samba shares (like ES File Explorer or File Expert for example), or any other samba client (like AndSMB). This way you could access shares on other computers on the network (but not share your own SD Card, you need a samba server on your device for this to work).