Construction of a 17c/45 spaceship (depicted below) for Conway's Game of Life was completed by Gabriel Nivasch on Dec. 31, 2004. It is a very large object; so far as I know, it is by far the largest nontrivial Game of Life pattern ever constructed. A lengthy computer program was written to generate the bulk of the spaceship.
The pattern is available in several formats:
A pattern this large requires a fairly heavy-duty Life program to display it. The quickest way to display it is probably to use the "hashlife" program mentioned above. It can read either the RLE or the MC format. However, it won't allow you to directly animate the pattern, and isn't particularly user-friendly.
The pattern can also be handled by (for example) Life32. On a typical computer, Life32 can animate it at about 2 frames per second, though it may take several minutes to load the pattern, and to advance from generation 0 to 1. Turning off all the Auto-snapshot settings beforehand may improve performance.
... Read Gabriel's article about this pattern. You could also look at my earlier (and out of date) document about it.
Gabriel Nivasch is primarily responsible for constructing the spaceship, and for several major improvements that allowed it to be made "small" enough to actually build. The overall design, and most of the engineering details, are by Jason Summers. The initial idea for the spaceship, and several of the important reactions, are by David Bell. The key 17c/45 reaction, which forms the basis of the spaceship, is ancient history.