If you remove a single player game installation with the Delete Local Content option from the Steam Libary, in general will save files be deleted or retained?
In general, the Delete Local Content option will remove everything from a game's folder located in the
steamapps directory except for the save files - they will be kept. There are a few exceptions to this, however, so take care.
In cases where anything is kept in an external directory, that will also not be removed. Many games nowadays have Steam Cloud enabled, which syncs your savefiles with an online database. This means that your save files will not only be kept, but synchronized with any computer you might want to play them on.
For Source games in particular, additional data will often be kept in more general Steam folders, too, and will then not be deleted.
Check if the game you want to delete has Steam Cloud enabled, and if not, check if the save data is kept outside of the game's
steamapps folder. If none of these are applicable, there is a very small risk that your save data will be deleted - this varies from game to game, though.
Depending on where the game involved stores the savegame files, Steam may or may not delete them. By deleting the Local Content, Steam is deleting the folders and associated files located in
steamapps folder. Those found under MyDocuments or other simmilar locations are left in place.
UPDATE: apparently it keeps any files that are generated prior to downloading.
On OS X, it is my experience that Steam won't delete save files. Most developers put their game's save data in
~/Library/Application Support/ as a standard practice on OS X, which mostly keeps them out of Steam's purview.
Now if a file was kept in
~/Library/Application Support/Steam/SteamApps/common/ (or wherever you happen to keep them) Steam will simply delete the whole game directory containing the game from the common directory regardless of whatever else is in there.
On OS X, as long as your game's saves aren't kept with the application, Steam will likely not delete them.