Garry's Mod
Garry's Mod, also known as GMod, is a sandbox Half-Life 2 mod by Garry Newman. It runs on the Source Engine, made by Valve
Bugs
When trying to connect to a server, you may crash somewhere during "Sending Client info..." If this occurs, open the Event Viewer, click "Windows Logs", then "Application" under it. Look through the folder and find the log that is an error and the source is "Application Error." Then click details and check to see if it says something like this under "EventData":
•c:\program files (x86)\steam\steamapps\(your steam username)\garrysmod\hl2.exe
•c:\program files (x86)\steam\steamapps\(your steam username)\garrysmod\garrysmod\bin\client.dll
If it does, open those folders where the files are located and delete hl2.exe and the client.dll. Then open Steam, right-click on Garry's Mod and then click Properties. Then click on the Local Files tab and click "Verify Integrity Of Game Cache...", wait for it to finish, then try reconnecting to the server you had trouble with. You should not crash this time. This does not work the first try every time. If you still crash, then follow the procedure again and keep trying.
Tweaks
It is possible to play the game in borderless windowed mode to simulate a fullscreen environment by right-clicking on 'properties' in your Steam Library, clicking on 'Set Launch Options...' and adding the following:
-noborder
While the game is set to windowed mode.
Installing Mods
To install most mods, simply extract the file downloaded to F:\Steam\steamapps\steamid\garrysmod\garrysmod\addons. Some mods, eg wiremod may require some more complex installation. ALWAYS check the readme for the mod before installing.
Links
Official GMod Wiki
Official Garry's Mod Blog
Official Garry's Mod Addon Website
Purchase Garry's Mod on the Steam Store
Console Commands
Games running on the source engine can utilize Source Engine Console Commands for various tasks.