You really can't say valve don't release sequels... Half Life, Half Life: Opposing Forces, Half Life: Blueshift, Half Life 2, Half Life 2: Episode 1, Half Life 2: Episode 2, Half Life 2: Lost Coast... not saying they're bad, quite the contrary, they just get better, but still I feel like being slightly argumentative.
Keeping on the theme of great sequels: Gears 2 is far better than Gears 1, Fallout 3 is by far the best fallout yet, Dawn of War 2 is far better than DoW 1, and hell move away from western games towards the far east...
Final Fantasy is on its 70 millionth game, and still going strong
Now I'm just going to list series that are after going through several follow-ups and are still going strong:
Quake, Unreal, Persona, Xenosaga, Street Fighter, Guitar Hero, Warcraft, Heroes of Might and Magic, Doom, Duke Nukem (even though we've not seen a new one in a while, you can't doubt the Mighty Boot), Dead or Alive, Need for Speed, Soul Calibur, Grand Theft Auto, Armoured Core, Dynasty Warriors
So, before you go on a 'SEQUELS ARE BAD GIVE US BRAND NEW GAMES ALL THE TIME' rant, (apologies for caps to those of us that are annoyed by those things, it's purely for effect) keep in mind that many sequels out there do things other than just act as free money (like, you know, every single sports game out there... do we really need a new Fifa every year? No! One every 3 years with patches to update player rosters would be fine).
Here's some examples of sequels that do what they do when they do what they do (modified quote by Turk from Scrubs... anyone get that?):
Final Fantasy - new universe every time (with 1 exception) with new characters and storylines
Unreal (Tournament) - Still one of the most fun multiplayer FPS games
Halo - While the 3rd one's story was sketchy, the multiplayer is the best thing about the game and it was needed on the 360.
Dead or Alive, Soul Calibur, Tekken, etc. - More characters, quicker, smoother, and in the case of DoA, worse breast physics
Guitar Hero - While I disagree with their current wave of spin-off packs and prefer Rock Band's huge amount of DLC method for getting music to the people, they added enough features to each of the main games (that is GH1, GH2, GH3 and GH:WT) to warrent being sequels
Grand Theft Auto - took a good thing (carjacking and shooting) and made it a better game every time
End note: There are some sequels that stand out in people's minds for different reasons, but they all have one thing in common: they take the series to a new level. GTA3 gave us a whole new type of GTA experience, Final Fantasy 8 and 9 had, by far, the best storylines to come out of Square (in my opinion before anyone argues it), Warcraft and Age of Empires started the RTS boom, Quake 3 and Unreal Tournament spawned voiceovers that are used everywhere these days, Gran Turismo 4 blew peoples minds with the amount of cars they could get into the game without negatively impacting graphics, gameplay, or quantity of other content, Krystal from Star Fox Adventures is commonly referred to as being the single source of 90% of the internet's furries (if you don't understand then you don't want to), Sonic & Knuckles + Sonic 3 (it's 1 game, if you don't get it then go do research on S&K) was just the best Sonic game ever.
EA and Ubisoft, as developers, have produced some bad sequels (mainly EA, I mean come on! *bashes Fifa again*), but as publishers they're not really at fault (as much as I love bashing EA). Take Command & Conquer, up until Westwood got absorbed into EA, EA's sole purpose was to publish the games and didn't have a hand in developing them. So please, learn the difference between Publisher and Developer before you go pointing your finger at companies and complaining while using excessive punctuation and overusing the words 'crap' and 'crappy'.