"Never look a gift horse in the mouth"
That's a G4 tower, built in mid/late 1999..
But don't worry--most 14yr old computers have already been long-recycled..
You could install OSX 10.4 - that's about as new an OS as I'd recommend.. you can get 10.5 on it with some work, but it's going to be REALLY REALLY sluggish...
that'lll never be powerful enough to handle modern web surfing (ie, youtube won't work), and won't really handle any modern software.. if you can get some REALLY OLD stuff, you'll have a great word processor, but little more.. Or you could load an old version of itunes on it and have a rockin music jukebox...