Hi Nick, welcome to the forum.
Hmmm, simulation games. Well Gma games produce two that I know of, Gma tank commander, and Lone wolf, where you command a 2nd world war Submarine. http://www.gmagames.com/ these games are commercial, but imho well worth paying for.
there are also a number of racing audio games around, but I'm not sure whether these class as sim or not. anyway, you can check out the free topspeed game from playing in the free topspeed 2 game from playing in the dark at http://www.playinginthedark.net/index_e.htm which also allows you to download extra cars and tracks, and race against other people online.
My personal favourite racing game is Rail racer from Blind adrenaline. Once again it's a commercial game, but has really exelent gameplay, as well as the ability to create your own tracks, buy upgrades for your racer and take part in online tournaments. sinse it's set in a sort of post appocalyptic future where the entire world has become blind, i'm not sure if it's sim or not, but if you like racing games I can deffinately recommend it. go to http://www.blindadrenaline.com/
As to text online games, the whitestick.co.uk site actually has an entire page devoted to them at http://www.whitestick.co.uk/ongames.html
I'd personally recommend Sryth, Kingdom of Loathing and Gothador (though Gothador can be a litle complicated to navigate). also, if your into fighting fantasy style gamebooks where you progress by clicking links, the Whitestick.co.uk site has a page for them as well, go to http://www.whitestick.co.uk/fantasy.html
Hopefully some of these links will be useful to you.
With our dreaming and singing, Ceaseless and sorrowless we! The glory about us clinging Of the glorious futures we see,
Our souls with high music ringing; O men! It must ever be
That we dwell in our dreaming and singing, A little apart from ye. (Arthur O'Shaughnessy 1873.)