Are there any interface mockup design applications for Ubuntu? I need one in order to design programs or interfaces (internet sites and more).
Software Recommendation – Best Interface Mockup Programs
software-recommendation
Related Solutions
How about hamster-applet? From the package description: Project Hamster helps you to keep track of how much time you spend on various activities during the day. Whenever you move from one task to another, you change your current activity in the GNOME applet. . It can present graphical statistics of how long you have spent on each task, and may be useful for project management or keeping employee timesheets.
StackApplet
Category: Other
StackApplet is a GNOME panel applet that monitors your activity on any StackExchange site.
stackapplet 
in Ubuntu Software Center
For Ubuntu 12.04 to 14.04:
Get the latest version as follows:
sudo add-apt-repository ppa:stackapplet-dev/stackapplet
sudo apt-get update
sudo apt-get install stackapplet
The first time the application is used, run
python /usr/share/stackapplet/stackapplet.py &
from a terminal.
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Best Answer
As has been suggested in many other similar threads, Pencil offers you a nice framework for prototyping and wireframing (even web stuff). And it's free.
I realise you've already said this isn't the answer you're looking for (having been linked to my question on the topic) but my scope is web development... Which is what you say you want to do. It's perfect for fleshing out page-level objects in a quick grid layout. Much more simple than a full graphics package, with various UI hints available as drag-on objects.
There's a deb package available for installation.