Whenever I get a new piece of equipment, it ends up being a prominent part of the sound for the particular album I am working on at the time. Very rarely do I use only one specific piece of gear for a song though; usually it is a combination of various pieces of equipment combined together.
House of Wires – Monogamy: was heavily sample based because I had just bought an Akai S20 and was going sample crazy.
Travelogue – The Art of Conversation: 90% of this album was done with synthesizers.com modules.
Travelogue – Telegraph: Much of this album is the Korg Mono/Poly. Some songs like Seasons are nearly all Mono/Poly.
Travelogue – Braille: The EML Polybox and various string machines (Moog Polymoog, Roland RS-09, etc) are all over this album.
Jon Sonnenberg/Israel Slick – Prometheus Project: On the songs of mine, which consist of half of this album, heavily used a Yamaha CS-40M and the EML Polybox.
House of Wires – You are Obsolete, Pivot Clowj (any of the albums), and Travelogue – The Noise is only Temporary: are free-for-alls where anything and everything in the studio was used.
Travelogue – Imaginary Hospitals: a free for all, but strictly analog synths.
Jon Sonnenberg – Acoustic Selections: a free for all, but strictly acoustic instruments.