[jason210]

In no particular order, these sims are what make having computers worthwhile for me. Of course, where applicable, all the best add-ons have been utilised.

FS2004
Silent Hunter III
Orbiter
Sub Command / DW
Operation Flashpoint
Aerowinx 744 Simulator
Pardox Games series (EUII, Hearts of Iron II, Victoria)

For instant, gratuitous feedback, the following I have succumbed to in the past:

C&C Tiberian Sun
Stronghold
Quake II
Close Combat III and IV

and more recently, for just plain fun:
Half-Life 2