There are three of them. They are all flight simulations, so if you have no interest in flying aeroplanes then I wouldn't bother. I assume you're at least aware of this, however, so...
Flight Unlimited is purely an aerobatics sim. Definitely showing its age these days, but mind-blowing stuff at the time.
Flight Unlimited II is a civilian general aviation sim (like MSFS), and so rather different to its predecessor. Unlike MSFS it's restricted to the relatively small San Francisco area, however all the terrain is from satellite imagery.
Flight Unlimited III is an enhanced version of Flight2, this time in the Seattle area, but you can also import the SF scenery from Flight2 if you have it.
If you have a special interest in aerobatics, FU is still pretty unique in the PC flight sim field. Otherwise, grab a copy of FU3 (See earlier thread on the recent re-release) and visit AVSim.com for aircraft and other add-ons.
MSFS2004 is easily the better looking civilian flight sim these days (it's had several years to catch up, after all), but many fans of FU3 still consider that the feeling of flight in Looking Glass' sim remains superior (and especially so if you're interested in gliders).
And at its best, FU3 still looks pretty.