@Mengine "Runtime checksum: 475de755cc276870468de509cfcdb91e steam-runtime.tar.xz, expected 247f17d2d019bf7ca1e2f19484ffa797 steam-runtime.tar.xz" is telling you that steam-runtime.tar.xz is damaged or corrupt, does the client launch if you delete ~/.local/share/Steam/ubuntu12_32/steam-runtime.tar.xz?

Deleted the file.
Now, a more detailed output is displayed:

Running Steam on ubuntu 13.04 64-bit
STEAM_RUNTIME is enabled automatically
Runtime checksum: 475de755cc276870468de509cfcdb91e steam-runtime.tar.xz, expected 247f17d2d019bf7ca1e2f19484ffa797 steam-runtime.tar.xz
Unpack runtime failed, error code 2
Error: Couldn't set up the Steam Runtime. Are you running low on disk space?
Continuing...
Installing breakpad exception handler for appid(steam)/version(1378500910_client)
Installing breakpad exception handler for appid(steam)/version(1378500910_client)
unlinked 0 orphaned pipes
Gtk-Message: Failed to load module "overlay-scrollbar"
Installing breakpad exception handler for appid(steam)/version(1378500910_client)
[0927/154131:WARNING:proxy_service.cc(958)] PAC support disabled because there is no system implementation
/home/buildbot/buildslave_steam/steam_rel_client_ubuntu12_linux/build/src/steamUI/../common/steam/client_api.cpp (290) : Assertion Failed: ClientAPI_InitGlobalInstance: InternalAPI_Init_Internal failed.

Assert( Assertion Failed: ClientAPI_InitGlobalInstance: InternalAPI_Init_Internal failed.
):/home/buildbot/buildslave_steam/steam_rel_client_ubuntu12_linux/build/src/steamUI/../common/steam/client_api.cpp:290

Installing breakpad exception handler for appid(steam)/version(1378500910_client)
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/assert_20130927154131_5.dmp
/home/buildbot/buildslave_steam/steam_rel_client_ubuntu12_linux/build/src/steamUI/SteamStartup.cpp (652) : Assertion Failed: ! "There was a problem with your Steam installation.\n" "Please reinstall steam.\n"
unlinked 2 orphaned pipes
CAsyncIOManager: 0 threads terminating. 0 reads, 0 writes, 0 deferrals.
CAsyncIOManager: 56 single object sleeps, 0 multi object sleeps
CAsyncIOManager: 0 single object alertable sleeps, 1 multi object alertable sleeps
[2013-09-27 15:41:31] Startup - updater built Sep 6 2013 12:04:22
[2013-09-27 15:41:31] Verifying installation...
[2013-09-27 15:41:31] Verification complete
Shutting down. . .
[2013-09-27 15:41:32] Shutdown

@Mengine so, it's the same error ... what happens if you delete steam-runtime.tar.xz, steam-runtime.tar.xz.part0, and steam-runtime.tar.xz.part1 then try to launch steam?

STEAM_RUNTIME is enabled automatically
Runtime checksum: 4e057d9c8c7c6e3d7de5117a90c0e176 steam-runtime.tar.xz, expected 247f17d2d019bf7ca1e2f19484ffa797 steam-runtime.tar.xz
Unpack runtime failed, error code 2
Error: Couldn't set up the Steam Runtime. Are you running low on disk space?
Continuing...
Installing breakpad exception handler for appid(steam)/version(1378500910_client)
Installing breakpad exception handler for appid(steam)/version(1378500910_client)
unlinked 0 orphaned pipes
Gtk-Message: Failed to load module "overlay-scrollbar"
Installing breakpad exception handler for appid(steam)/version(1378500910_client)
[0927/155428:WARNING:proxy_service.cc(958)] PAC support disabled because there is no system implementation
/home/buildbot/buildslave_steam/steam_rel_client_ubuntu12_linux/build/src/steamUI/../common/steam/client_api.cpp (290) : Assertion Failed: ClientAPI_InitGlobalInstance: InternalAPI_Init_Internal failed.

Assert( Assertion Failed: ClientAPI_InitGlobalInstance: InternalAPI_Init_Internal failed.
):/home/buildbot/buildslave_steam/steam_rel_client_ubuntu12_linux/build/src/steamUI/../common/steam/client_api.cpp:290

Installing breakpad exception handler for appid(steam)/version(1378500910_client)
Uploading dump (out-of-process) [proxy '']
/tmp/dumps/assert_20130927155428_5.dmp
/home/buildbot/buildslave_steam/steam_rel_client_ubuntu12_linux/build/src/steamUI/SteamStartup.cpp (652) : Assertion Failed: ! "There was a problem with your Steam installation.\n" "Please reinstall steam.\n"
unlinked 2 orphaned pipes
CAsyncIOManager: 0 threads terminating. 0 reads, 0 writes, 0 deferrals.
CAsyncIOManager: 55 single object sleeps, 0 multi object sleeps
CAsyncIOManager: 0 single object alertable sleeps, 1 multi object alertable sleeps
[2013-09-27 15:54:25] Startup - updater built Sep 6 2013 12:04:22
[2013-09-27 15:54:25] Verifying installation...
[2013-09-27 15:54:25] BVerifyInstalledFiles: ubuntu12_32/steam-runtime.tar.xz.part0 is -1 bytes, expected 52428800
[2013-09-27 15:54:25] BVerifyInstalledFiles: ubuntu12_32/steam-runtime.tar.xz.part1 is -1 bytes, expected 45046872
[2013-09-27 15:54:25] Verification complete
Shutting down. . .
[2013-09-27 15:54:28] Shutdown
Finished uploading minidump (out-of-process): success = yes
response: CrashID=bp-02f30050-09eb-4d58-a3bc-4e0652130927

hmm ... so the cover-all solution is to run steam --reset from a terminal, this will re-bootstrap the client as if it is the first time steam is run on your computer. That includes re-downloading the client, and it will leave your games alone.

Its working again :)

Thank you very much :+1:

Closing as resolved, thanks @Tele42 :)