I've separated lines by material, and indicated where the company that produces them is based. If a line is roughly compatible with either the classic Necromunda 3" square bulkheads or the new Sector Mechanicus 5" tall walkways, I've indicated that as well. I've excluded lines which are purely small-scale detail pieces, as I would be here until judgment day listing all the suppliers of resin accessories. Most publishers have several lines, not all of which will be strictly "Necromunda-esque"
If you know of any that I've missed, let me know!
Games WorkshopUK with international distribution. Sector Mechanicus line is modular. But you knew about them already.
Mantic Games Battlezones.UK with international distribution. Based around 3" squares connected by tabs. Industrial sci-fi theme. Extremely cubic.
Pegasus Hobbies as Hexagon, Chemical Plant, Syberclicks. People may know it as "the Robogear scenery" as it was a pack in with the Airfix-distributed Robogear game. In theory you can buy the sprues in bulk direct from Tehnolog (and the line is slightly more extensive than is distributed in the west) but none of their affiliate sites seem to have an e-commerce portal.
Lasercut / CNC milled
Shiny Games. Lasercut MDF. Really detailed Neo-tokyo style range, and post-apoc American-urban range.
Complex Red (budget, styled on classic cardboard Necromunda terrain.)
Hardfoam stuff - the stacked boxes are a good hill alternative for an urban setting.
ToysThere are loads of kids toy kits that make great terrain with a bit of work.