Phaedrus said:
Complete Warrior
Complete Divine
Book of Vile Darkness
Book of Exalted Deeds
Unearthed Arcana
Eberron
Draconomicon
Fiend Folio
MM 2
As everyone pointed out, it depends a lot if you are going to use the book as a player or DM, if you want quantity material or something to give a striking renovation to your character or campaign, and if you actually want to use the book or just read it
1) Book of Exalted Deeds
This book gives really a new edge IMHO. Playing characters who don't want to kill or destroy is much more difficult than one can think, and BoED provides very goo material. An exalted campaign is much more different from typical D&D than a vile campaign is, so this is the book to buy if you want to seriously try something new, although you can also cherrypick the book as any other player supplement if you don't want to fully embrace the exalted concept
2) Unearthed Arcana
As I wrote in my review, this is the most long-term investment book around. Some variants are so strong that they can change the game completely (especially the magic variants). Again, this is a great book if you are looking for something very new, although it will require a good effort from the DM and the players to get used to the changes.
However, this book also has a lot of charater material which you can just pick for your PC. It is completely amazing how many people say "don't buy UA because you are not going to use it... buy Complete X instead"

. It's just plain wrong. UA has for example very many class variants, just as the Complete books have many prestige classes, definitely it doesn't have as many, but on the other hand the character material in UA easily generates many more combinations, while PrCls for example are very static.
There is absolutely no reason why a DM should allow the Complete Warrior/Divine base and prestige classes and disallow the UA variant classes!
3) Eberron
I am not going to buy this setting, but from what I have heard it is quite an innovative setting. If you are the DM and you are looking for something "new, but not too much new", take a look at the many preview on the Wizard's site and decide if this setting appeals you.
4) Draconomicon
This book is definitely for the DM. Yes, it also has character material but not much and not versatile. To the DM it provides different kind of aids when using dragons in your adventures, not just new material but also charts and samples of existing material.
However, the best thing of the Draconomicon is that is it a pleasure to read

even if you decide not to use much of it.