Steam Chat
Steam Chat (formerly Friends and originally Tracker) is a instant messaging feature on Steam, which allows players to chat with friends.
It was later updated in July 2018, with it's UI redesigned using CEF, and adds new features (Group Chat, Rich Presence, Emoticons (emojis), Voice Chat support, and more) that were compared to Discord. Steam Chat can be also accessed through web browser.
In Valve engines and previous Steam versions
When Steam Beta was launched in 2002, and with the 1.1.0.9 update for Half-Life (and other GoldSrc games), which at the time rely on WON (World Opponent Network) service, it adds VGUI and Tracker support. Most users at the time did not use Steam Beta and install Steam Tracker so players will see "Tracker Error: TrackerUI.dll invalid, not loading" appearing on the Developer Console.
When Steam was launched, alongside with the first Steam build of Half-Life (in 2003), and the release of Half-Life 2, when it was known as Steam Friends, the chat UI was based on VGUI2, and was very barebone (lacking many features). You can add friends, set status, edit profile, and play Parlor Games (Checkers, Chess, Go, Hearts and Spades) with friends.
In older version of 
GoldSrc and 
Source (Source 2004 and Source 2006), there is a "Friends" menu option which opens up Friends window, as Steam Overlay wasn't present back then.
Both engines (and Steam) had two versions:
- Friends v1 - The first version features Parlor Games. This version of Friends no longer works as it relies on deprecated Steam API or Steam server which has been offline. Both GoldSrc and Source, between (December 2005 to January 2006), Friends feature was disabled in this version, when Steam transition from Friends v1 to v2.
Bug:Parlor Games option in Source does not include any options to play Checkers, Chess, Go, Hearts and Spades.
- Friends v2 - The second version (introduced in January 30, 2006), also referred as Friends Beta. This version removes Parlor Games, but much of it was identical. It still works with latest Steam client (from 2025), but it was buggy (chat doesn't show up in-game, but does on Steam client), this is because of some Steam API features are deprecated because of security reasons and/or simply outdated. Both GoldSrc and Source engine gets updated to this version when it was released in January 30, 2006.
Friends are eventually removed in all GoldSrc and Source games in July 31, 2007, as Steam Overlay (which was likely released around August 6, 2007 with the introduction of Steam Community) replaced many of it's functionality. Despite Half-Life 2 and Half-Life 2: Lost Coast continues to run on Source 2004 (and last updated in April 2006) until Half-Life 2 was upgraded to Source 2009 with the release of Mac version of the game (in 2010), and Half-Life 2: Lost Coast upgraded to Source 2013 during SteamPipe, the Friends feature was retroactively removed, possibly through .gcf files (on Steam) overriding GameMenu.res with the one from .gcf that removes the option.