Ubuntu One service will definitely stop on 1st June, 2014: it is time to look for a replacement !
Here is a list of solutions with my choices and remarks. I did not test them all, for most of them, it's only information from their web site.
Do not hesitate to post your choices in the comments
Google Drive: doesn't have any official applications on Linux, and Google is not keen on confidentiality. Not really interesting.
Microsoft OneDrive: obviously does not support Linux ... and not famous for confidentiality either !
BitTorrent Sync: the solution to not use a server (good confidentiality), hence no limit ... but you better always have a machine up and running (otherwise no backups) !
Seafile: own cloud or hosted on Amazon (1 Gb for free), multi-platforms.
SparkleShare: no smartphone client, hosted by one self, versions files, open source.
ownCloud: well integrated, and extra applications as well as services (calendars etc). One must create its own cloud !
It's a bit complicated ... os there's a all in one solution that uses the distribution Syncloud as it is described in anarticle from Korben (in French).
There are also hosts providing the solution ownCloud, for instance in France la mère Zaclys.
The Why not
Dropbox: the oldest, not much space (2 Gb by default), many platforms, only one directory (can be worked around with symbolic links on Linux). Note: everything is encrypted on Dropbox so your data is fairly safe.
The extra from Dropbox: files versioning ! One of the rare few to do it (with Spider Oak) !
Sync: multi-platform (not yet on Linux but "soon"), versions files, crypted data, 5 Gb for free.
Tonido: multi-platform. Works either on master/slave (without any cloud) or uses filecloud. Quite unusual.
BitCasa: multi-platform, free for 20 Gb et 3 machines and encrypts data.
Wuala: comprehensive, on most of platforms, servers in Switzerland, only 5Gb for free
(from Lacie, my favourite external hard drive company)
Copy : bien intégré dans Linux, espace honnête (15 Go), possibilité de partager le poids d'un fichier (1 fichier de 10 Go partagé entre 4 utilisateurs ne comptent que pour 2,5 Go à chacun), fichiers encryptés. Présent sur la plupart des plateformes.
Tresorit : multi-plateforme, versioning, encryption, serveurs en EU, sûr (récompense au hacker qui trouvera la faille), 5 Go gratuit (jusqu'à 16 Gb avec des bonus) - pas de versionning en gratuit.
The Winner (at the moment)
HubiC: OVH offer. Used with command line. Multi-platform, 25 Gb for free.
I choose it because it's hosted in France, its hosted size (25 Gb, the biggest), it works on Linux and it's not too dificult to set up and then invisible to use ... and I'm alreay an OVH customer.
I shall see how it goes.
Here are my choices do not hesitate to post yours in the comments.