The Underworld trilogy consists of the original Underworld movie (2003), Underworld Evolution (2006) and the prequel Underworld: Rise of the Lycans (2009). Underworld 4 is set for release in 2011. *squee*
Kate Beckinsale certainly seems to have a penchant for vamps or tight corsets, I'm not sure which, so you can also catch her in Van Helsing starring alongside sexy Hugh Jackman (if only he wouldn't wear that hideous hat). Those of you who have read Dracula will get the Van Helsing references made in the movie.
But back to Underworld: One of my favourite vampire movies because it's set in the 21st century with all the high tech crap that nerds like myself enjoy, plenty of fun weapons and a dark storyline (not so much really). The original Underworld is superior to the prequel (rise of the lycans) in both storyline and filmography. Most notably the third movie is very dark (literally) and hence is difficult to see. I can't help but wonder if the budget was smaller so they were trying to skimp of special effects. It's quite annoying.
Underworld I: Sexy, leather-clad heroine with a chip on her shoulder expertly slays lycans. 4/5
Underworld II: Sexy, leather-clad heroine and tag-along, tasty, hybrid, manbag fight the originals. 3/5
Underworld III: Not as sexy, leather clad heroine in love with somewhat attractive underling lycan, a sad tale which you'll know the ending to from watching the first two movies. Sadly lacking in storyline because it's already been spelt out in the previous movies, they still could have done better. 3/5