(Here follows an almost verbatim copy of a self-answer from an identical question on serverfault which I'd forgotten about; askubuntu wasn't yet created).
Based on information found in this page about enabling XDCMP and the file /etc/gdm/gdm.schemas, I managed to create a /etc/gdm/custom.conf file:
# /etc/gdm/custom.conf
[xdmcp]
[chooser]
[security]
DisallowTCP=false
[debug]
Take care with letter case: it won't work, if you write "disallowTCP=false"...
I also changed the /etc/X11/xinit/xserverrc file to:
exec /usr/bin/X11/X
i.e. I removed the -nolisten tcp options to the X executable. I don't know if I needed to. You might want to try avoiding this edit.
If you only change the xserverrc file, X will nevertheless start with "-nolisten TCP".
After that, all that is needed is a restart of the gdm process:
sudo service gdm restart
You can verify the success as:
tzot@tzot-laptop:/etc/X11
$ netstat -an | grep -F 6000
tcp 0 0 0.0.0.0:6000 0.0.0.0:* LISTEN
tcp6 0 0 :::6000 :::* LISTEN
Update
After an upgrade to 12.04, I had the same issue. This time, the culprit is the lightdm that the system uses. The file that needs to be updated is /etc/lightdm/lightdm.conf and the required addition is a xserver-allow-tcp=true in the [SeatDefaults] section.
And afterwards, I also found that answer. :)
Synopsis
So, in 10.10 this still works: create /etc/gdm/custom.conf with contents as specified above and restart gdm.
On each computer install the ssh package.
Then you can mount the filesystem over SFTP (stands for Ssh File Transfer Protocol) in nautilus and copy that way just like you would between drives. To access files on the other computer, open nautilus and Clik the File -> Connect to server... menu. Pick ssh from the menu/list. Put in your username for the other computer and the IP for the other computer and click connect. It should ask for your password for the laptop username you put in and then it should show the files on the laptop. (You can do this in either direction.)
This method encrypts the whole thing both ways so it is safe even over open wifi (or if you use strong passwords) even over the internet.
You can find out the IP of a computer by running ifconfig on it. It will give you something like this:
eth0 Link encap:Ethernet HWaddr 00:11:11:9e:58:57
inet addr:192.168.1.15 Bcast:192.168.1.255 Mask:255.255.255.0
inet6 addr: 2001:2038:243:de16::f4d1/64 Scope:Global
inet6 addr: fe80::237:41ff:fe9d:5237/64 Scope:Link
UP BROADCAST RUNNING MULTICAST MTU:1500 Metric:1
RX packets:2874340 errors:0 dropped:0 overruns:0 frame:0
TX packets:317014 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:348505532 (348.5 MB) TX bytes:127490351 (127.4 MB)
lo Link encap:Local Loopback
inet addr:127.0.0.1 Mask:255.0.0.0
inet6 addr: ::1/128 Scope:Host
UP LOOPBACK RUNNING MTU:16436 Metric:1
RX packets:68246 errors:0 dropped:0 overruns:0 frame:0
TX packets:68246 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:0
RX bytes:35179159 (35.1 MB) TX bytes:35179159 (35.1 MB)
Look for a line that says 'inet addr:192.168.###.###' (that does not start with 127.0.) and that is the IP address. So for the example ifconfig output I listed, it would be 192.168.1.15 This would be the address I would use to connect to the computer I ran ifconfig on.
Best Answer
You may not be on the same WORKGROUP as your other shares - so a list is unable to be generated.
Try connecting directly to the share via it's IP address and protocol to ensure you're still able to see and reach any share.