A tragically overlooked game, and, in my opinion, the greatest survivor horror game of all time.
Don't forget to watch in HD!
A brief overview of starting character customization and in-game skills in general
As the game begins, you're given a choice between three military branches: The Marines, the Navy, and the OSA, aka psychic black-ops. The last of the three sounds the most awesome, certainly, but each one is worth considering in its own right as they all have very distinct play styles.
The Marines are primarily focused on weapons training. They start with one tier in Standard weapons, learn how to repair and maintain weapons, and, depending on your choices, you can even start the game with a grenade launcher or a laser pistol. If you want to make the game play as closely to a standard FPS as possible, this is the path for you.
The Navy, on the other hand, focuses more on the technical side of things. While they also start with one tier in Standard weapons, their starting skill choices include hacking various computers and security devices (very handy), repairing and modifying weapons, and even researching enemies to discover their weaknesses. If you like a more technical approach to things, apply to be a sailor.
The OSA's main focus is on developing and using psionic abilities. These more or less function as this game's "magic", enabling you to do all sorts of fun stuff you wouldn't be able to normally. Launching ice and fire from your hands, becoming invisible, removing your mental safeguards to make yourself inhumanly strong temporarily, and so forth. OSA characters start out with the first two psychic power tiers unlocked by default, as well as a few first-tier powers available for immediate use. The downside is that they aren't nearly as well trained in any other aspect - they don't even have Standard weapons start out with, and no real technical skills to speak either (unless you pick the option in the second choice that gives you a point of Research). Still, having the second tier unlocked right away does give you a significant head-start over the other two branches, as it costs a whopping 26 Cyber Modules just to enable it for use on the other two paths. So if you like the idea of using crazy superpowers and don't mind being poorly equipped and barely trained in the early stages, the OSA can be very fun and rewarding.
SS2Tool - Patches the game to run much more smoothly on Windows XP and above (and enable mods).
Anomalies, Discrepancies, and Outright Bugs (Beta 0.3.0) - Fixes a few design flaws in the game, as well as making a handful of minor tweaks to weapons and armor to better balance the gameplay.
SHTUP Beta 6 - Cosmetic, replaces many textures with higher-res versions to give the graphics a cleaner look.
Arcaniac Graphics Pack Reduxed - Cosmetic, replaces most of the character models with nicer looking versions.
Rebirth 02 - Cosmetic, replaces several enemy models with nicer looking versions.
SHMUP - Cosmetic, replaces the stock music tracks with higher quality versions.