It all depends on what facet of the job you're speaking of. Comparing Marine Force Recon to the SEALS is tough, but i'll give it a shot.
In the Selection process, where the hundreds of applicants are tested and eliminated until they get the handful they need, nothing really compares to the SEALS selection and training program (the acronym is BUDS, i forget what it stands for), that's where the expression "Hell Week" originated. Hell Week ain't no joke either, think of a 6-day period where you get perhaps 5 hours of sleep the entire time, and are constantly on the move, in the water, or hauling around a Zodiac landing boat.
However, once you get into the unit, my judgement is that the Force Recon guys have the harder task. No matter what the branch of service, Army, Navy, Marines, etc. LRSD (Long Range Surveillance Detatchment) teams have the hardest job in all the military. SEALS have a bit of glamour thing going on (not uncommon with elite units) and get some of the more high-profile and cream puff missions, while the Force Recon guys i've met still think of themselves as just knuckle-draggers, gun bunnies, ya know, regular Marine types, and just keep on doing SR missions (Special Reconnaisance), which is one of the toughest, although most common, type of Special Operations mission.
<< Are there still forces called the Green Beret? >>
Yep, although the official terminology is "US Army Special Forces." The green beret is the headgear we wear (well, past tense in my case), although some folks use it as a term to describe the soldier himself.
<< what other special forces are in the United States military? >>
Well, true "special forces" in the military would be defined as those which require selection to be a member of (as opposed to the so-called 'elite forces' such as the 82nd Airborne, 101st Air Assault, etc., which are regular line units, open to all, but with specialized missions). Some examples would be:
US Army Special Forces (the 'green berets')
US Army Rangers (75th Ranger Regiment)
US Army Special Operations Air Regiment (160th SOAR)
US Navy SEALS
USMC Marine Force Recon
USAF Special Operations Command units
More that i can't think of from the top of my head
Also, on an individual level, there are quite a few folks holding specialized duties, who while technically not "Special Operations qualified", might as well be. These folks go out and do very challenging and dangerous jobs, and often have riskier missions than most 'true' Special Operations soldiers. Combat contollers would be a prime example.