I thought that people here might like to hear about a game that I really like.  That would be SWAT 4.  It's a first person shooter, but it simulates a number of realistic SWAT situations.  You play as an elite police officer, with a squad of officers and a variety of modern weapons at your disposal.
 
After some basic training, you will experience a variety of exciting situations.  First, you will experience a team briefing, where you can examine interesting mission data (such as maps and photographs of suspects).  Secondly, you get to equip your squad accordingly.  There is a large variety of weaponry, which includes assault rifles, various crowd control grenades, sub machine guns, pistol sidearms, C4 breaching charges, gas masks and helmets.  Once you're satisfied with your selection of armaments, you will commit to the mission and experience a SWAT mission from a first-person perspective.  Depending upon the circumstances, you must infiltrate a dangerous situation (often involving hostages), and then neutralise or arrest the suspects.  This is often very difficult, as the suspects may be heavily armed and quick on the draw.  Once you have "brought order to chaos", you will receive a debriefing, and the option to proceed onto the next mission.
 
One of the most interesting aspects of the game is the 'use of force'.  As a police officer, you are expected to handle each situation with utmost professionalism, which means not opening fire with a deadly weapon (such as my favourite, the Colt M4 Carbine) unless yourself or someone is in immediate life-threatening danger.  In essence, you must enter a room, and assess the dangers present.  Usually, you will first need to announce your presence (pressing the "F" key will result in your character yelling something akin to "Police!  Hands in the air!").  Next, you will need to take appropriate action, depending upon what's happening.  If a suspect raises a deadly weapon to attack, you are authorised to open fire to stop the suspect.  However, if they surrender, you are expected to arrest them (pressing the "H" key will equip zip-cuffs, allowing you to restrain the suspect).  If you meet with a suspect who is non-compliant, you may need to employ a less-lethal method to force compliance.  This may be as simple as a punch, or it may involve use of a tear-gas grenade, a taser shock, a blast from a bean-bag shotgun, or a squirt of pepper spray to their face.  You'll also need to collect any weapons they drop, and announce any arrests made over your radio.  This strict adherence to police procedures makes this game very unique, and exciting.
 
Another interesting factor lies in your team mates.  You can order your fellow officers to perform actions.  This may, for example, allow you to perform simultaneous entries into a room from multiple doors, giving the element of surprise over your enemies.  You can also order your squad mates to cuff suspects while you cover them.
 
The graphics are quite beautiful.  Even though this game is a few years old, you'll not find it too shabby.  The locales that you visit all have an authentic feel to them.  There are dirty restaurants crawling with cockroaches and clueless customers, dingy houses inhabited by serial killers, and even professional office buildings with frightened employees.  If your PC is no more than a few years old and has an nVidia or ATI graphics card (not an Intel onboard graphics array), you shouldn't have any worry playing this game.
 
The audio of the game is also quick involving.  You'll hear terrorists shriek when blinded by a flashbang, you'll hear doors blast open with a loud bang when C4 charges are employed, and you'll even get to listen in on realistic 911 calls in your briefings.
 
Altogether, I think that this is an excellent game, and certainly worth playing if you're a fan of SWAT units and law enforcement.  A few of the earlier games in the SWAT series may have been lacking, but I found SWAT 4 was good fun and very exciting.
 
If you like the sound of this game, you can read some reviews at:
http://au.gamespot.com/pc/action/swat4/review.html
http://au.pc.ign.com/articles/600/600178p1.html

Quote from Hondo I. Sackett on Jun 18^th, 2008, 10:35pm:

It's been out since 2005, with an expansion pack in early 2006, so you should be able to get it pretty cheaply.  Here, in Australia, I can buy it for about $50 in a pack with the expansion pack, which is fairly decent value.  I'd imagine that it would be much cheaper in the US, though.