Vampire The Masquerade: Bloodlines
Bloodlines is an underappreciated action/roleplaying game with a vampire theme. Your character, himself a vampire, must complete a series of quests while not drawing attention to his undead status (termed breaking the Masquerade). In doing so he gains experience points that can be spent improving his skills and abililites.
If you liked Thief and Deus Ex you should definitely give Bloodlines a look.
As one of the first games developed with Valve's Source engine, Bloodlines inherits some early bugs which are fixed in later iterations of Half-Life 2 and other titles. One particular annoyance is that it will crash displaying an error message about having less than 15Mb free memory when run on 64-bit versions of Windows with over 2Gb RAM installed.
This bug was subsequently fixed in Half-Life 2 but the developers of Bloodlines never incorporated the fix into their game.
I first came across the problem in January 2006 after I upgraded to a 64-bit system and left it alone after reading various people complaining about the bug and deciding there was no solution.
In August 2006 I was bored and decided to take a crack at the problem. It turns out that I was successful (why else would this document exist?) and so I did some more research to see if there was a FAQ entry somewhere on the internet I could post to. Amusingly I then found that a few other people had simultaneously come up with solutions ranging from the silly (restart Windows with a flag telling it not to use all the RAM available) to very similar to what I did.
In the interest of completeness I now present my solution to the problem.
Just skip to the end to download the files if you aren't interested in all the technical stuff.
Updated 2006-12-06 as various sites seem to be referring people to this article when they have trouble with the game.
Updated 2007-08-30 with detailed instructions for people unfamiliar with hex editing.
Updated 2007-09-20 with a patcher to fix the game automatically.
Updated 2007-10-25 with separate patchers for v1.0 and v1.2. Thanks to Kevin Smith for assisting with creating these patchers.
Updated 2007-10-26 with patchers to fix the installer.
Updated 2010-04-24 with notes on the low-resolution textures issue.
Updated 2011-08-02 with notes on patching the Steam installation of the game.