rakarrack is a decent effects rack for linux.
It's not packaged for ubuntu(of course), but is fairly easy to compile. I've been using the cvs version (yes, people are still quite happily using cvs it seems).
Once you get it set up you have access to 80 presets of realtime guitar effects(2-40ms depending on your computer sound card setup).
It works with the jack audio system, so you can route audio into it, and route its audio out to other programs easily enough. You can also control it with midi(alsa, or jack). Meaning you can hook it up to a midi controller of some sort. I've been using it with an maudio axiom 25 controller, and with python scripts via pygame.midi.
It even has a very nice help manual integrated into the program... I wish more programs had a good help section. It details the 17 different effects that come with the program.
One thing which is a bit funny is that it doesn't use LADSPA or lv2 plugins. You can of course still use these other plugins through other jack programs if you want. However the ones it comes with are all fairly good quality from my tinkering. There is a lot you can do with the various plugins in different orders with different settings in each one. The 80 presets show a good variations of sounds you can get out of it.
I've been using it in conjunction with the mixxx mixing program and the hydrogen drum machine. So you don't need to use it just with guitars... any program or audio input will do.
If you are using guitars it also has a tuner which might come in handy... if you've never quite got the hang of playing that song called Tuning that you hear at the start of every gig.