suptg is a /tg/ resource site run by Lord Licorice for the benefit of all. It is most well known for the thread archival functionality it provides, although the site also provides a number of other functions, such as image storage, online game listings, and even a Paintchat and IRC server. The site is located at http://suptg.thisisnotatrueending.com If the site is not responding, this means that Lord Licorice was using the server to cook lunch, but it overheated.
If you're planning on archiving a thread, PLEASE read Lord Licorice's criteria for a quality thread and do not post your Touhou image dump or 4e troll thread, or else we'll all hate you and he'll probably delete your contribution.
The archival page has voting and sorting capability, efficiently allowing to access only the best threads and eliminating the necessity of creating a "best of archive" link-dump anywhere. It has also had the unfortunate side effect of turning thread quality into a popularity contest, and the ease at which threads can be archived with false names and tags has made it of dubious worth, especially due to its reputation for being useful only for quest threads. Also, Lord Licorice and several other big cheeses on sup/tg/ are goons.
LL and a few minions are currently in the process of purging the archives of art dumps, unrelated (or bad) threads and failed quests. And nothing of value was lost.
If you like a thread, but it doesn't meet the standards of Suptg, or for some tragic reason stayed under Suptg's radar, it is probably on https://warosu.org, http://desuarchive.org/, or 4Plebs, who automatically archive everything posted on certain boards. However these websites save everything, its probably not worth the time it takes to find it unless you have an idea of what you are looking for and can find it with a search tool. If you have nothing better to do than sift through the low standard (un)dead threads of
foolz archive.moe desuarchive looking for nothing special in mind, then you need to discover more productive and/or fun things to do. Unless of course you are just looking for pics, but in that case you ought to just start a new thread, or look for an art thread (with the search tool of course).