On my server, the /var-volume (in fact, the logical device /dev/vgroup/var, part of my LVM), is running slowly out of space. Therefore, I decided to extend it by 5 GB.
Fortunately, with reiserfs, you can do this while the volume is mounted – at least, this is what is said in the LVM-howto.
So, here’s what I did (after doing a full backup):
lvextend -L+5G /dev/vgroup/var
resize_reiserfs -f /dev/vgroup/var
This went extremely well and fast – I was really (positively) surprised! 
