Steven Wilson is the vocalist, guitarist and producer for the prog-alt-metal band Porcupine Tree as well as projects such as Blackfield, No-Man and Bass Communion. He's also famous for working with the who's who of prog-metal acts, such as Opeth.
In terms of guitars, Wilson predominantly plays Paul Reed Smith guitars, including PRS Custom 22's, Singlecut and the Modern Eagle. In the studio he has also been known to use Gibson Les Pauls in the studio. Before the release of the In Absentia album, he was often spotted playing a ESP Stratocaster.
In the studio Wilson is said to rely quite a lot on the Line 6 POD for a lot of the guitar tones. On the Fear of a Blank Planet album he is quoted as saying that he also used the Bad Cat Hot Cat, as well as a Diesel head, straight into a generic 4x12 Celestion Greenback Marshall cab mic'ed with a 58 and a 414. He's also a big fan of plug-ins, as he points in a MusicPlayer.com interview:
There’s a suite of plug-ins [Digidesign’s D-Fi collection] of which one is called Lo Fi, which is fantastic. I love Lo Fi – it’s the kind of thing that Trent Reznor uses a lot where he gets those really fizzy digital f**ked-up sounds. It basically enables you to take a sound, not just a guitar sound but any sound, and reduce the bit rate and the sample rate until you get that kind of wonderful quantization noise. Well, it’s not always wonderful, but it can be wonderful used in the right way, so I used that a lot to get really fizzy kinds of sounds, almost digital distortion sounds, so that’s one of my favorites.
I love the Line 6 Echo Farm, which is the old kind of tape delay simulator – that’s wonderful. I’m using that all the time. I like being able to add things like warble and tape hiss and stuff to the signal – it just makes it sound more organic somehow. And, I’m a big, big fan of the Focusrite EQ and the Focusrite compression, the D2 and the D3. They’re, I think, probably my favorites.
In terms of playing live, his onstage rig has changed a bit over the years. Where once he used to play through a substantial effects rack, he has now gone to a more stream lined floor based system, as a result of the cost implications associated with lugging around the rack units. As you can see from the picture of one of his pedal boards below, his rack gear has essentially been replaced with the TC Electronic G-System effects unit. There are also a couple of Boss volume/expression pedals as well as what looks like a Dunlop Crybaby. Although it is hard to make out a couple of the pedals, this board also seems to feature a Carl Martin compressor, Boss pedal (perhaps the Boss Distortion?), Bad Cat two tone distortion and what could be a GigRig midi switching system.

The second board pictured below again includes the GigRig switching system as well as an Ernie Ball Volume pedal, Boss DD-20 Delay, Boss RT-20 Rotary Twin, Boss Chromatic Tuner, Analog Man Sun Face and what looks like a Dunlop Crybaby Wylde Wah. There are three other pedals on there as well, but I can't make them out. One is a Boss pedal of some sort, with four knobs, so that rules out a Boss DS-1.

Just as an added bonus, here's a short clip of the making of Fear of a Blank Planet.
If you've never heard of the Porcupine Tree before, I'd highly recommend picking up In Absentia
, one of my favourite albums of all time.