I don't know for absoluetly shure if this topic might be a repeat of an existing one, only thing I found casually looking through was recomendations for abadonware sites. What I have in mind for this thread is other kind of preservation: not about where to get copies of files of certain games, but more about how to run old games on modern machine. Emulators, tools, fan patches, tweaks, and probably most cool: opensource fan ports of game engines. Currently, on a modern PC, you can run basically almost any games, but sometimes people don't know how. Maybe we can collect here at least some information and offer some help to make the process of playing old games more enjoyable.
Maybe I will start from some usefull basic links:
1. PC gaming wiki is a usefull place to check if there any fan patches, fixes or twaks avalible if you not shure they exist or can't find them casual way.
2. Very awesome list of OpenSource game clones, ie new engines for old games made by fans. Big list of projects - saldy, not full, sometimes not most updated but still, great source to look into.
3. A Wiki about emulators. Also may be not super-detailed in places, but follow updates and development of many emulators, have descriptions, recommendations and lniks to them,
4.The Patches Scrolls is nice resource to search for the patches for old DOS games. If you install them from floppies or from CDs they most often will be not the latest version.
5.VOGONS is forum community about running old games on newer systems. One of the first places in the internet where you can ask direct question about topic about running old games and fix their issues, especially if you not found solutions in other places