As it is popular among postcrossers to collect stamps about Postcrossing, here is a list about all the official ones. Also here is a list about special postmarks that are connected to Postcrossing. Both lists are continued from the old forum.
The Lists:
The List of "official" Postcrossing Stamps
“Official” means it is approved by Postcrossing and issued by postal/government bureau.
2011
October 14 PostNL (The Netherlands) image
2013
September 9 Posti (Finland) image
2014
January 2 Belpochta (Belarus) image
May 28 Guernsey Post (Guernsey) image
2015
January 25 Russian Post (Russia) image
May 29 Pošta Slovenije (Slovenia) image
September 2 Česká Pošta (Czech) image
October 9 Ukrposhta (Ukraine) image
2016
March 29 PostNL (The Netherlands) image
May 21 Austrian Post (Austria) image
July 14 Polish Post (Poland) image
July 14 Guernsey Post (Guernsey) image
2017
January 3 Belpochta (Belarus) image
February 24 Romania Post (Romania) image
July 14 Indonesia Post (Indonesia) image
September 7 Swiss Post (Switzerland) image
October 4 An Post (Ireland) image
2018
February 2 Magyar Posta (Hungary) image
June 23 Poșta Moldovei (Moldova) image
2019
June 7th Åland (Åland) image
2020
July 14th Correios (Brazil) image
2021
June 10th Guernsey Post (Guernsey) image
July 10th Belpochta (Belarus) image
July 14th Austrian Post (Austria) image
2022
September 13th Post Luxembourg (Luxembourg) image
October 1st Eesti Post (Estonia) image
October 6th Deutsche Post (Germany) image
Appendix:
Stamps issued by an postal administration, but not approved by Postcrossing: Bulgaria 2015, Donetsk Peoples’ Republic 2021, Kazakhstan 2015 and 2020, Lithuania 2013, Malaysia 2017 and 2019 (Pos Silang), Russia 2016
The List of Special Postmarks
All of these are independent ones, so they are not associated to any Postcrossing stamp.
2012:
Taiwan: Taipei Meet-up
2013:
Taiwan: Taipei Meet-up
2014:
Czech Republic: Prague Meet-up (second picture, apparently it was used on multiple days of the Sberatel Stamp Fair)
Poland: Warsaw Meet-up
2015:
Germany: 10 years Postcrossing
Indonesia: Medan Meet-up
2016:
China: Wuxi Meet-up
Czech Republic: Prague Meet-up
Finland: Helsinki Meet-up
Malaysia: Kuala Lumpur Meet-up
Poland: Wroclaw Meet-up
Russia: Postcard Exhibition in Perm
Taiwan: Taipei Meet-up
USA: 1st SW Meet-up
2017:
Åland: Mariehamn Meet-up
Czech Republic: Prague Meet-up
Malaysia: Kota Kinabalu Meet-up (image needed!), Kuala Lumpur Meet-up
Moldova: Chisinau Meet-up
Poland: Wroclaw Meet-up, Frankfurt - Słubice Meet-up
Switzerland: Stamp Exhibition in Sindelfingen
Taiwan: Taipei Meet-up
USA: 2nd SW Meet-up
2018:
Åland: Mariehamn Meet-up
Azores: Angra do Heroísmo Meet-up (better image needed!)
India: Bengaluru Meet-up
Indonesia: Denpasar Meet-up (image needed!)
Moldova: Chișinău Meet-up
Poland: Wrocław Meet-up, Katowice Meet-up, Poznań Meet-up in September, Poznań Meet-up in October, Bydgoszcz Meet-up
Portugal: Lisbon Meet-up
Singapore: Meet-up
Taiwan: Taipei Meet-up
USA: 3rd SW Meet-up
2019:
Åland: Mariehamn Meet-up
India: Coimbatore Meet-up (Postmark 1, Postmark 2)
Indonesia: Denpasar Meet-up (image needed!)
Moldova: Year-long Postmark in Chișinău
Poland: Poznań Meet-up in May, Krakow Meet-up, Lodz Meet-up, Torun Meet-up, Zielona Góra Meet-up, Elblag Meet-up, Poznań Meet-up in September, Wroclaw Meet-up, Krakow Meet-up
Taiwan: Taipei Meet-up
USA: 4th SW Meet-up
2020:
Åland: Mariehamn Meet-up (Meet-up postponed, postmark was reworked, first version)
Austria: Hirtenberg Meet-up (Meet-up cancelled, but the postmark was available from the postmarking centre)
Mexico: Mexico City Meet-up
Poland: Oleśnica Meet-up (Meet-up cancelled, but the postmark was available at the local post office), Wrocław Meet-up
Taiwan: Taipei Meet-up
USA: 5th SW Meet-up (meet-up was postponed, but the postmark was used anyway)
2021:
Poland: Two Year-long Postmarks in Wrocław, Wrocław Meet-up
Taiwan: Year of the Ox Meet-up in Taipei
USA: 6th SW Meet-up
2022:
India: Bengaluru Meet-up (image needed!)
Italy: World Postcard Day Meet-up in Camerata Cornello
Luxembourg: Luxembourg Meet-up
Used for longer than a Year:
Belarus (available in 18 post offices, complete list posted below)
Russia: Common Motive 1 (Cheboksary, Gelendzhik, Kazan, Krasnodar, Krasnoyarsk, Novosibirsk, Oryol, Samara, Sochi, Sterlitamak, Tyumen, Ufa, Yekaterinburg, Use began in 2015 or 2016, Blagoveshchensk, Khabarovsk, Use began in 2018), Common Motive 2 (Michurinsk, Tambov, Use began in 2018, Belgorod, Use began in 2019),Moscow (Use began in 2015), Kirov (Use began in 2016), Arkhangelsk (Use began in 2017), Chelyabinsk (Use began in 2017), Izhevsk (Use began in 2017), Kaluga (Use began in 2018)
More Information needed
Belarus: Two postmarks from Minsk (Were similar postmarks used in other cities? Are they still in use or permanent? Did they replace the other postmark?)
Ukraine: Chernivtsi (any information needed! Permanent? Still in use?), Kiev (Is it permanent or was it used for an event?), Vinnytsia (second Picture, did was definitely used for more than a year, but when did it start and if yet when did it stop?)