You can write a letter or some postal cards to me (I love them), at the following addresses (listed by decreasing preference):
The Mathematics department secretary, at ENS de Cachan, in France (you can see it on this OpenStreetMap map)
Mr Lilian Besson, ENS de Cachan, Dept. de Mathématiques, 41, avenue du Président Wilson, 94230 - Cachan, France
to my place (in France), but this adresse is for private use only.
If you are a spammer, go on: jarvis.spam@laposte.net! Oh, and, go fùck yoùrself also.
If you are not a spammer, go on, you can use this one:
Lilian dot Besson at ens-cachan dot fr
To speak directly, but only if it is really important, you can try to call me, you could use my French cellphone number:.
Avis
I am currently out of the French territory, living in Switzerland (in Lausanne).
Calling someone in a foreign country can be expensive! In fact, even receiving them is costly, so please avoid it (if possible).
You could also try to send me text messages (SMS), with the same number, obviously (again, but be aware of the price!).
The next solutions are way more geeky, and you will need a relatively recent computer to try them (or a smartphone).
These two pages can be used to ask my a question publically (everyone can see what you ask and my answer): on GitHub : github.com/Naereen/ama, github.com/Naereen/ama.fr (requires a GitHub account); and one Bitbucket bitbucket.org/lbesson/ama, bitbucket.org/lbesson/ama.fr (no need for an account).
From time to time, I appear on the IRC network irc.crans.org, in the rooms #crans and #general, with pseudo Naereen (I usually use irssi with these configuration files, in a terminal, obviously).
If you are feeling particularly audacious, you can try to "let a message for me" there, that's really a bad idea.
You could follow this procedure (in French) to call me: call the French fix number +33.9.72.11.32.70 then compose "one four zero four two" (14042) when the robot asks you to do it.
You could also try to call me (or to text me) at the Linphone account: naereen at linphone dot org (" at " = "@", " dot " = ".").
Note
Linphone ?
Linphone is a free and open-source voice over IP implementation, which works perfectly. They have a desktop application (for Windows, Ubuntu etc) and one for mobile phones (Android etc). For Android, I prefer to use the awesome CSip Simple. Linphone also provides free user accounts, to call (without limit) any other Linphone users, thanks to VoIP and the SIP protocol.
These web services, strongly based on the great functions offered by the HTML 5, offer the possibility to talk, to write or even to share files, dynamically.