MTP

MTP (Media Transfer Protocol) is a protocol to allow the transfer of files to external devices. It is provided by several programs, most of them depending on FUSE.

Kernel
Prepare for MTP implementations by activating CONFIG_FUSE_FS in the Linux kernel.

Available software
Here are some MTP packages available in Gentoo:

KDE SC 4
kio-mtp is a slave for KDE's KIO framework. It is still unstable, but it is in the main repository.

First, install:

Quit KDE, and (re)launch it again. Now the MTP devices should appear in Dolphin.

KDE 5
kio-extras contains MTP support, but it requires the  USE flag.

Enable the global  use flag:

Alternatively, enable it in kio-extras:

Then emerge:

After this, quitting KDE and restarting is probably sufficient to get it working.

If that does not work, please try restarting the system.

gMTP
gMTP is a simple MTP client for Solaris and Linux.

It is sufficient to just install and run :

Click on connect. A few second later, the file hierarchy on the device should appear. It is now possible to manipulate the files on the device. It support features like Album Artwork and playlists.

Troubleshooting

 * If the device is not recognized by libmtp, try upgrading to latest (or even live) version - (presently v1.1.12).
 * Some ROMs (e.g. PAC ROM) require the unlocking of the screen of the device before files will appear. If the screen is not unlocked the device will appear empty.
 * Sometimes (specifically on HTC One X) USB debugging automatically activates when device is connected to a PC. USB debugging must be disabled, otherwise libmtp can't recognize device.
 * Make sure the Android device is not going to sleep and the screen is not getting locked. Set screen timeout to very long values, or enable in Development section flag "Do not turn off the screen".
 * Lumia devices might require version 1.1.9 and  to be mounted properly.
 * Lumia devices must be unlocked to be able to connect (enter the user's PIN).