Installation for maps:

..\Valve\Steam\SteamApps\[your user name]\half-life 2\hl2\maps\[map name.bsp]

Note that some installations, such as downloaded copies of Steam, may not include the "Valve" directory.

Command Line:

"..\Valve\Steam\Steam.exe" -applaunch 220 +map [map name] +skill 3

Skill: 3 (hard), 2, 1 (easy). Note that the -applaunch id may be different for non-English versions of the game. Check this page for the correct number.

"C:\Program Files\Valve\Steam\Steam.exe" -applaunch 220 +map leonHL2_1 +skill 3

Note that -console can be added to the command line to enable the console, -novid can be added to skip startup videos and +sv_cheats 1 can be added to enable cheats. See this page for more command line options.

Maps for Episode 2 are installed to the ..\half-life 2 episode two\ep2\maps directory. The shortcut is as above, but the applaunch id is 420.

"C:\Program Files\Valve\Steam\Steam.exe" -applaunch 420 +map union01 +skill 3

OR for mods (indicated by below):

..\Valve\Steam\SteamApps\SourceMods\[mod folder name]

The mod should then appear in your Steam games window. Shortcuts to Source based mods can be made by right clicking on the mod name and selecting "Create desktop shortcut". A shortcut will then be created on your desktop, which you can then move to your start menu or other location as desired.