Warped Gravitational Field Propulsion can be doable according to one theory - Electrogravitics
There is also an idea for Buoyant Saucers too, and I know Russia has them and America has them too (of note is that a saucer is not the only shape for buoyant craft, triangles or hexagons or cigar shaped blimps, or any other shape is equally valid). The idea buoyant saucers might simply be that they float to the upper atmosphere and then launch away using ... nuclear propulsion or some other propulsion means.
There is also a lightcraft design for flying saucers and the coanda effect (or electro-hydrodynamically too)
Then of course there is also the Noisy Hexacopters and Graceful Quadrotors too, which can easily be mistaken for flying saucers...
Here are some links to UFOlogy I've looked into so far -
Vortices and Victor Schauberger
1984 Whiteman Airforce Base Incident
Dan Akroid on UFO's (8 part documentary)
History Channel - Ancient Aliens
Here are some Daily UFO Headlines
Intelligent robotics, of course, is something that would allow significantly longer lifespans than organic life intelligence has. Hence, SETI now aims to search for robotic lifeforms in the universe as well, which opens up a whole new frontier (asteroids are for all intents and purposes, indistinguishable from orbiting spacecraft of similar size on radar, or perhaps there are stealth spaceships completely invisible to NASA's radar too) as far as the possibilities for intelligent drones and self-replicating spacecraft goes, there seems to be no limit.
