Topex/Poseidon was launched 30 years ago today (August 10, 2022). If the data began to be measured later on in 1992, this mission's launch marks the birth of a time series whose continuity and consistency is of foremost importance for a number of applications. The iconic application when thinking "long term" is of course the global Mean Sea Level. A number of others have been listed is these pages (non exhaustively - a number of other applications, missions, datasets exist).

1992. 1992 Topex/Poseidon was launched on August 10 - the result of about 10 years of work on both sides of the Atlantic, and the beginning of a new era in oceanography and remote sensing.
1993. 1993 The first data analyzed from T/P data were represented and released as "quick-looks", later made as leaflets and posters
1994. 1994 The first 'El Niño' of the 30-year period. Not by far the strongest, nor the first seen by an altimeter, but the first one that T/P ever observed.
1995. 1995 During Hurricane Opal Topex/Poseidon monitored high winds and significant wave heights; there were discussions on the impact of a warm eddy on its intensity, too, so the interest of SSH on hurricane intensity forecasts was discussed. 
      the Mercator project was launched on that year, where the SOAP project was running
      the FES95 tide model began assimilating altimetry data
      first lake monitorings from T/P
1996. 1996 Altimetry SWH data were assimilated in Météo France "VAG" model
1997. 1997 Strongest and most prolonged El Niño event of the 20th century (cont'd in 1998)
1998. 1998 A Mean Sea Surface was computed from altimetry dataset (released 1999)
1999. 1999 La Niña -- strong and prolonged, too  (cont'd in 2000)
2000. 2000 Argonautica educational project launched with, for the first year, skippers of the Vendée globe around-the-world alone sail race releasing Argos-tracked buoys for students to monitor, and altimetry maps plotted to analyze those buoys's paths.
2001. 2001 Jason-1, launched, the "T/P Follow-on".
2002. 2002 Topex/Poseidon was gradually transferred to a new, 'interleaved', orbit, as all its successors have been when a new mission was calibrated and declared fit for service.
2003. 2003 A Mean Dynamic Topography from altimetry (among others source data) released
2004. 2004 The tsunami in the Indian ocean observed by coincidence by several altimeters, helping to improve tsunami wave propagation modelling
2005. 2005 Mercator Ocean first global ocean forecasting bulletin at 1/4°
2006. 2006 An El Niño popped out
2007. 2007 Icebergs detection around Antarctica, eddy tracking algorithms and Finite-sized Lyapunov Exponent, all computed over the whole time series
2008. 2008 Jason-2 launched
2009. 2009 Jason-1 transferred on an interleaved orbit
2010. 2010 One of the strongest La Niña. Some coastal products began to be generated and disseminated.
2011. 2011 Quirin storm. 20,1 m SWH recorded
2012. 2012 Beginning of the Jason-1 Geodetic Phase: non-repetitive orbit to measure on a tighly-knit mesh of tracks
2013. 2013 Saral launched, the first "Ka-band" altimeter and a French-Indian cooperation
2014. 2014 El Niño played 'hide and seek' in 2014, demonstrating that more monitoring and studies are needed to better understand the phenomenon, and accurately forecast it. The following year stressed this need, with a strong El Niño staying in the middle of the Pacific Ocean (El Niño "modoki")
2015. 2015 Altimetry multi-mission data (Duacs) operationally disseminated in Copernicus Marine Service
2016. 2016 Jason-3, launched. Later the same year Jason-2 was transferred on an interleaved orbit
2017. 2017 GPS carpet, a new tool to calibrate altimetry satellites
2018. 2018 CFOSat launched, the first of a new kind of sensors, focused on SWH measurements, a French-Chinese cooperation
2019. 2019 American great lakes very high level recorded
2020. 2020 Sentinel-6 Michael Freilich launched
2021. 2021 High flood on the Garonne river
2022. 2022 30 years of Topex/Poseidon, and soon Swot launch (also: Jason-3 moved on an interleaved orbit)