Here is the thing.
I've got a TP link router (TL-WR541G) and 3 PCs accessing to Internet through it.
2 Desktop PCs (1 with Debian and 1 WinXP)
1 laptop (with Ubuntu).
In my router (192.168.1.1) I can see the MAC and IP of all machines assigned (DHCP) by the router.
But when I ping to them (from any box to any other) I get a Destination Host Unreachable. How can I view my others machines in the WLAN? What am I doing wrong?
Sorry if this is too "newbie" 
any help is welcome
Your windows machine probably won't respond to pings.
Your debian machine might not respond to pings unless configured to do so.
Do your other machines show up in your arp cache after you try to communicate with them? /sbin/arp -n If so, you are communicating with them, with arp, and they're just not responding to pings, and other communication should work.
Thanks! It works! arp -n shows me the others addresses. So theoretically I can leave any port open (using nc for example) in the any machine and access through the others. Is this correct?
Another question.. What the best way of sharing sources in this network? Is there any alternative to Samba?
Samba is the thing for windows. It can talk to it natively. It can even verify passwords and permissions through it.
For occasional ad-hoc things you might like udpcast. It's a general commandline stream tool like netcat, but it broadcasts, so you can have multiple receivers. It has a windows utility too.