Not so much a mod as a map pack with a new story, :Causality Effect: A Red Letter Day Gone Wrong” is well worth the 30mb download for anyone who considers them self a Half-Life fan. The game starts off in a familiar setting, the beginning of the Red Letter Day chapter of HL2. You go through the normal routine, talk to Kleiner, Barney, Alyx, get your HEV suit, and play with the cactus teleporter by the door for about 10 minutes. After this you get into the teleporter and surprise surprise, Lemar decides to mess things up. You have a rough teleport, but instead of ending up outside of the lab, you end up inside, but eight hours into the future.
Stepping out of the teleporter you find the lab full of combine, and Dr. Breen is accompanying them. They kill Kleiner, Breen leaves, and the game starts. You fight your way through about an hour to an hour-and-a-half of quality HL2 mapping, with brief story moments ever 15 minutes or so when you trigger a teleport through time by walking into certain rooms. I won’t give it away, but the games alternative story is quite suitable for a map pack. My favorite part of the story was that they recorded no additional dialog, and instead used only existing HL2 phrases to put together something completely different- quite astonishing given the limited selection. With four different endings, and a reasonable difficulty, I see no reason not to recommend this.
So if you’re bummed about HL2: Episode 2’s constant delays, hop on the mod train and check out Causality Effect