Rifts is easy to get into once you get past character creation. There is a fairly large skill set system and the magic system is MP based. It can bog down if you do combat actions the way that it says in the books. A few people that look at the system get game paralysis on character class/ racial class/ psychic class choices. It's similar to Pathfinder in that way as it has a huge amount of variability. Once you are past initial choices though it is all percentile based on skills.
GURPS and Champions can both fit the bill as well. They are both worse on character creation though than Rifts. GURPS especially so. GURPS is similar to Rifts in that it is anything goes. Champions is Superheroes, but it does have support for just Sci-fi settings if you ramp up the tech level and use magic as "Psionics".
4th(?) Edition Gamma World is pretty good, but it uses 2nd edition AD&D rules. It was a pretty good edition though. Simple to play, lots of customization and zany adventures.
Morrow Project is near-future, So there are psychic powers and laser weapons along with power armor. Mostly though, it is a paramilitary setting about finding your place in the world.
Corvus Belli is putting out a new game soon, it is Called Infiniti. It is based off of a miniature game.
In the same spirit as that one, Gear Krieg was pretty sci-fi for being set in WWII. It was about super-science.
On the zanier side:
For Savage worlds there is the Low Life supplement where you can be a intelligent cupcake.
HoL - an old White Wolf (published under Black Dog I think) game where you are stranded on a wasteworld where there is no plot and everything is permitted.
Darwin's World - never played it, but it looked funny.