The most well-known examples are, of course, Moot and Jim Watkins' testimonies under oath, but there are probably many more floating around.
* '''4chan''' - Despite its notoriety, only two major court cases mention 4chan, one in passing but one with a subpeona calling 4chan founder Christopher Poole (Moot) to testify, thereby recording 4chan's existence in court precedent for time immemorial.
** [https://www.courtlistener.com/opinion/621755/united-states-v-kernell/?q=4chan&type=o&order_by=score+desc&stat_Precedential=on United States v. Kernell, 667 F.3d 746 (6th Cir. 2012)] & [https://www.courtlistener.com/opinion/2469889/united-states-v-kernell/?q=4chan&type=o&order_by=score+desc&stat_Precedential=on United States v. Kernell, 742 F. Supp. 2d 904 (E.D. Tenn. 2010)] - When a 4chan user was arrested for hacking into Vice Presidential Candidate Sarah Palin's Yahoo.com email account, given that the user discussed, leaked to, and was goaded upon by other users of the site, the court deemed it necessary to issue a subpeona to Christopher Poole (Moot) under pain of oath as a witness to describe the website and terminology in full.
* Imgur - Imgur is more commonly seen in court proceedings as a witness as it hosts images which is subject to far greater liability than reddit text.
** [United States v. Morel 2019 DC District Court] - Establishes that "private" imgur albums linked nowhere are not considered private and exempt from unreasonable search and seizure, as it is detectable by google crawlers and by the third party custodian itself as per its terms and conditions: which as a legacy reference still mentions digg at such a late era.
** Sinclair v. TubeSockTedD 2009 - A mention of digg as a forum that the comment disparaging the user was posted on, but no further information about the site was recorded.