What interface mockup programs are available?

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.

As was mentioned above WireframeSketcher wireframing tool is also available on Ubuntu. Recently I started providing a standalone version so it's not Eclipse only anymore. Give it a try.

I see you mention "internet sites". For web interface mockups, I like to use gomockingbird.com.

It's very quick and web-based, so it will definitely work in Ubuntu. You could probably use it for software application interface development, as well.