I love my high-end tools as much as anyone but there are some good games/engines out there that don't require extreme technology (and budgets) to play or use as development platforms. Game with more modest resources, i.e. on par with the typical business PC, have the advantage that your simulation can reach a much broader audience. These are also good tools to use if you are trying to develop a concept simulation and can't justify yet a high-end development or run-time platform yet.
Here are some of my recommendations:
Return to Castle Wofenstein / Enemy Territory - This game is based upon an older version of the Quake engine. The latest version, Enemy Territory: Quake Wars, will use the latest Doom 3 / Quake 4 engine which is pretty resource intensive but RTCW/ET plays very well on a modest PC or laptop. Enemy Territory is a free download so you can't beat the price. Both use the Radiant tools which are also the basis for many other games so you get lots of mileage from your learning investment.
Quake III - This is one of the most heavily modified games ever. id has announced they will make the game code open source. You can work in Quake III at the code level or use the Radiant tools to modify/create your own levels. By current standards, this game has very modest hardware requirements and provides a rich toolkit. Also uses the Radiant tools so you can "grow" into other games/engines later. Even though Quake4 is the latest version, there is still a very active community around this tool
Counter Strike - Out of the box it is a great starting point for tactical and law enforcement sims in the FPS genre. Have to hunt around some but there is lots of good documentation on the web on how to use the tools. A bit of a technology dead-end though since Valve switched to the Source Engine. Still, even though CS:Source is the new iteration of this product, this version is still actively sold by Valve and very actively used within the community.
Counter Strike: Source - A rebuild of the original but to use the Source engine. Despite using the Source engine, you can "dial down" the graphics and effects to make this run on a more modest PC - more so than you can with Half-Life 2 which has become more of a showcase product (especially with the episode add-ons). The one resource you can't get around is a fair amount of disk space. CS:Source, Half Life 2, etc all share the source engine on your PC - additions don't add much but that first application brings along a couple of gig of disk space with it. You can dial down the CPU, memory and graphics requirement but can't get around that initial multi-gig storage hit.
Call of Duty - COD2 added a smarter AI, vastly better particle effects, and upped the CPU/GPU/RAM requirements. The original COD is more manageable. Tool support is really just for the map editor and not really for the program logic. Some of your tool skills will scale up to COD2 but not all. Investing in a product that is no longer sold would be concern.
Unreal Run-time - This is free and can be downloaded from the Unreal Developer network. Has very modest resource requirements compared to the Unreal/editor you get in the games (e.g. Unreal Tournament 2004). Your learning investment scales well from this product to the editor in UT2004 et al. A main way it saves on resources is that you get almost nothing in the way of assets (textures, shapes, sounds, etc.) Assets are what makes UT2004 take up so many install CDs (and so much disk, CPU, memory, etc.) The Run-Time version is smaller footprint but it this means you have to build up so much stuff from scratch which might be a problem. Very well documented - UT2004/editor documentation (including the 3DBuzz videos) can be applied pretty easily as well.
Medal of Honor franchise. - Based on the older Quake engine so modest resource requirements and you can use the Radiant tools that scale well into other products. Comes with more modern assets (shapes, textures, sounds, etc.) so a better starting point than the space/fantasy content in Quake. Despite using the older engine, still an active part of the EA product portfolio so availability should be good for awhile. Not as active a developer community as for Quake though.
Delta Force games by NovaLogic - You can't edit the maps and you can't edit the logic but does include a decent mission editor that has some level editing features. At the bare minimum of what I consider a development option - good more for concept development than simulation. Still the mission editor is decent and assets real-world so not bad to lay out some ideas. This is not a big player in the market - so your learning investment wouldn't take you far in what is available in the industry.
SWAT 4 (Gold Edition) - Despite modest resource requirements on the box, the original version seemed to have much higher actual resource requirements. I've heard though that despite the great game-play there were ongoing bugs in video drivers that are fixed in the latest "gold" edition (which bundles the original with an expansion pack). No code access and no map editor but a decent mission editor. This one is a "maybe" - I can't vouch given the history with the original but it might be an option.
Halo (PC) - Microsoft Studios has almost always made sure most of their products can reach a very large number of machines and not just to the hardware extreme. Halo is no exception. That has some consequences in features for sims though - Face Poser for Half-Life 2 is extremely powerful and useful but nothing similar exists for Halo because the faces in Halo are either very simple or hidden by visors. The product configuration is a bit odd also in that you can only make levels for an add-in multiplayer environment for Halo - not the out-of-the-box game.
Empire Earth II - Here is an RTS favorite. Much more modest graphics and physics than Empire Earth III. Has a map tool but (to my knowledge) no way to re-write the application logic. Still, a bit of a product/technology dead-end since EEIII is out. Only a modest community out there for this version.
Rise of Nations - Comes with a map editor and a script editor for editing the logic of the game. The scripting environment is specific to this platform but uses an event-driven paradigm that is widely used elsewhere - conceptual you get some mileage out of that investment. You can create scenarios that are modern - good for simulations - but sadly this product has been replaced with Rise of Legends which has a fantasy theme at odds with serious simulations.
Warhammer 40,000 - This is another "maybe" and more of a candidate than a recommendation due to my lack of hand-on time with this engine/tool. The game as a very good RTS engine in it and Relic clearly knows its stuff in this genre. The Relic developer site looks informative and has lots of tutorial and map-making information. The space/fantasy theme is an obstacle but looks like that could be overcome in a mod without too much difficulty. Has an active mod/developer community. Again, a "maybe".
NeverWinter Nights - My only RPG recommendation....primarily because I know less about RPGs. NeverWinter Nights has been used for several studies or experiments discussed in journal articles from the IEEE, ACM, etc. Even though NWN2 is available "any day now", this version will likely have an active community for years to come. If you buy the DVD version you get lots of product for your money. Tools are easy enough for school kids to use.