ownCloud

First make sure you have your RAMP set up correctly. Then it's really simple:

1) Follow steps described here to install latest version of ownCloud using apt-get (without it, my RaspBMC downloaded and installed ownCloud 4, while the latest at the time of writing this is 7.something):

sudo apt-get install owncloud

by default main ownCloud's files should install in /var/www/owncloud/ (including config/config.php), also apache's config file pointing to that location should be placed in /etc/apache2/conf.d/owncloud.conf (editing this file should allow you to change ownCloud's default URL)

2) Create directory where all your data will be stored, and make sure it's owner is changed to the user that apache runs on:

sudo chown -R apache_user:apache_user_group directory/

should do the trick

3) Complete ownCloud setup by launching http://your-rpi-local-address/owncloud

(here I stumbled upon some problems which I managed to solve with that fix)

4) Finish by polishing your ownCloud's configuration file located in /var/www/owncloud/config/config.php and edit trusted_domain section to enable access from internet (only if it's your intention!):

'trusted_domains'=>
array (
0=>'your-internal-domain-or-ip_address',
1=>'your-external-domain-or-ip_address'
),

5) WebDAV should be enabled by default, so use it with the following URL:

internal-or-external-domain-or-ip/owncloud/remote.php/webdav

Bunch of useful links:
http://www.ebayram.net/how-to-install-owncloud-on-ubuntu-and-debian/
https://forum.owncloud.org/viewtopic.php?f=8&t=18742
http://www.instructables.com/id/Raspberry-Pi-Owncloud-dropbox-clone/step3/Configuor-php-Apache/
http://doc.owncloud.org/server/7.0/admin_manual/installation/installation_wizard.html