If you are familiar with GuitarPort, GNUitar is almost the equivalent. No expensive input device is needed though, this one is free, open source and works on Windows and Linux. Simply plug your guitar into your soundcard (line-in works best), you may need to buy an adapter to make it fit. You can add several effects and adjust each one as you please. The effects are: distortion, sustain, reverb, echo, delay, chorus, flanger, equalizer, noise reduction, wah-wah, phasor, tremolo and vibrato.
The delay time in Windows is like half a second or so. Their website says the delay in Linux is much shorter. I haven’t tried it on Linux myself though. Some times the sounds sound too computerish but that could be just my soundcard and settings I use. Overall its a nice program and fun to play with.