I've maintained an unreasonable interest in these things for a long time -- over a decade by now, it appears. That linked article talks about five interchangeable types of signals, with explicitly constructed stable G-to-X and X-to-G converters.
If we count the *WSSes separately, as we probably should, that was eight stable-circuitry signal types known in 2007 -- glider, LWSS, MWSS, HWSS, Herschel, 2c/3 diagonal wire, swimmer.
Here's an updated list, which I'll try to keep updated based on any future improvements, or reminders about existing technology, that show up on this or other threads.
Work in progress. More links and dates to be added. Donations gratefully accepted.
Up-to-date re: replies in this thread as of Towel Day 2019.
Spaceships, from Goldtiger997's list:
most recent: Snark and syringe, Apr 2013 and May 2015
most recent: G-to-LWSS Oct 2015, codeholic's 45-degree L|MWSS-to-G reflector Oct 2015
most recent: G-to-MWSS June 2017, H-to-MWSS, G-to-MWSS and alternate G-to-MWSS Oct 2015, codeholic's 45-degree L|MWSS-to-G reflector Oct 2015, Sokwe's 135-degree MWSS-to-G reflector July 2013
most recent: G-to-HWSS Apr 2017, HWSS-to-G Oct 2015, HWSS-to-H repeat time 407, August 2018
- universal *WSS-to-G
repeat time 497, Oct 2015
- escorted OWSS
recipe and p120 gun by Dieter Leithner, 5 November 1994, no converter constructed yet
- Big A
recipe and guns, August 2009, no converter constructed yet
most recent: G-to-sidecar and sidecar-to-HWSS Apr 2017, sidecar-to-G May 2016
- 2-engine Cordership
glider-to-Cordership repeat time 556, August 2018, and repeat time 458, September 2018
- 3-engine Cordership
most recent: G-to-Cordership Feb 2017, Cordership-to-swimmer Oct 2008.
- 4-engine extendable Cordership
most recent: Adjustable glider-to-Cordership, previous discussion,
- 6-engine Cordership
most recent: G-to-6-engine-Cordership (in progress), 6-engine Cordership eater and Heisenburp Cordership-to-G Nov 2016
- Coe Ship
most recent: G-to-Coe, Coe-to-LWSS and Coe-to-G Apr 2017
G-to-ecologist March 2018
- space rake
glider-to-space-rake mentioned in passing, completed April 2018
most recent: G-to-loafer May 2018, G-to-loafer July 2017, G-to-loafer Mar 2017, loafer-to-B and loafer-to-G March 2013
most recent: G-to-25P3H1V0.1 and 25P3H1V0.1-to-G Mar 2017
partial glider synthesis, Dec. 2017
most recent: G-to-weekender-to-G Feb 2017
- 2c/5 spaceship
recipe and gun, February 2016
most recent: crab-to-G May 2016, G-to-crab May 2019.
25-glider synthesis Jan 2016
13-glider synthesis, Mar 2015
- half-X66 with HWSS
half-X66 with HWSS to clean HWSS, March 2018
- Pushalong 1
77-glider synthesis, June 2015
65-glider synthesis, Jan 2015; partial p480 gun Apr 2017; complete p960 gun; no glider-to-30P5H2V0 yet.
85-glider synthesis, Sep 2015
- Pufferfish spaceship
[60-glider synthesis Feb 2015 is best effort so far, opportunity for variable length/min population...]
most recent: glider-to-copperhead August 2018, copperhead-to-glider April 2016
most recent: glider-to-fireship March 2019, fireship-to-glider April 2016, composite fireship-to-copperhead June 2018, previous composite fireship-to-copperhead June 2017.
most recent: spider-to-glider converter, March 2019, some potential Heisenburps in previous post; glider synthesis of spider March 2019
G-to-46P4H1V0 converter, October 2019
50-glider 27P4H1V1.1 recipe, October 2019
64P2H1V0-to-G October 2019.
-- let's just leave G-to-those-monsters as "technically possible"...!
- Sir Robin and minstrels:
Sir Robin to glider, with or without attached minstrels: Goldtiger997's Amazing 2 in 1 Reusable Multipurpose Minstrel Remover and Detector™ -- which currently supports "all minstrels that are made up of only errant, lightweight, heavyweight, wandering, and pi minstrels".
glider to Sir Robin: not even vaguely on the far horizon yet, except on April Fool's Day
- lightspeed telegraph (orthogonal beehive wire)
G-to-lightspeed, lightspeed-to-G -- see Calcyman's stable-telegraph.mc in Golly's Online Archives > Very Large Patterns
- 2c/3 diagonal signal wire
G-to-2c/3 September 2017, 2c/3single-to-2c/3double and 2c/3single/double-to-G (decades old, in Golly 3.1+); modernized 2c/3single-to-2c/3double and 2c/3single/double-to-G July 2018, mostly by Goldtiger997.
- c/2 diagonal signal wire
140 gliders to c/2 signal, October 2018; partial recipe provided for c/2 signal to glider
- c/12 swimmer
G-to-swimmer and swimmer-to-G Mar 2017. See also swimmer-to-G Apr 2009, convertible to swimmer-to-H by adding a Snark.
- G-to-X via highway robber
951-tick Competent Highway Robber G-to-G, and also 886-tick Awkwardnessful Highway Robber Gang G-to-G-to-G (has to work in teams of two) -- March 2015. Repeat time 878 found by chris_c a little later, rewired by Entity Valkyrie to avoid the need for a second highway robber.
- pseudo-Heisenburp G-to-X
- stable B-heptomino tracks by Guam Dec 2011, simsim314 Apr 2015, AbhpzTa July 2016 (no Herschels)
- pi heptomino conduits
no stable signal loops currently known
- traffic jam signal loops
p25N loops known; stable Heisenburp for passing traffic jam April 2017, glider-to-TJ April 2019.
- R (R-pentomino)
- B (B-heptomino)
- H (Herschel)
- C (century)
- P (pi)
- Q (queen bee)
- W (wing)
- G (glider)
- G0, G2, G4, G5, G6, G13, etc. (various tandem-glider signals, see the Elementary Conduits collection)
- D (dove)
This seemed so promising that it even made it to the list on the first page of the Elementary Conduits thread, but I don't think we quite have a workable X-to-D and D-to-X yet (?).
four different stable glider-to-5c/9 converters, but there isn't any known way to get a signal out of a 5c/9 wire, with the nearest miss being via a 2c/3 wire, but that would be very slow and expensive even if it was repeatable. Also some other unproven converters scattered around the Elementary Conduits thread.
Some potential conduits with E-heptomino inputs and outputs are known, but nothing that allows a signal to get to an E and back again. Short way of saying that: there's no known X-to-E-to-X.