There are great deals to be done in the mid-season transfer window; here are our top ten.
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Virgil van Dijk (Southampton to Liverpool, 2018)
Many felt the Anfield outfit had overpaid for the commanding centre-back but within months he was looking like a bargain. The Dutchman transformed the Reds' erratic defence and helped them to the UEFA Champions League final. This season, Liverpool have shipped just ten goals in their first 21 league games.
Kevin De Bruyne (Chelsea to Wolfsburg, 2014)
De Bruyne spent just one full season at Wolfsburg, but needed no time at all to make an impression. The Belgian scored 16 goals in all competitions and returned a record 21 top-flight assists in 2014/15 as the Wolves finished second and won the German Cup for the first time. His efforts brought him the German Footballer of the Year award and a move to Manchester City.
Ivan Rakitić (Schalke to Sevilla, 2011)
The Swiss-born Croatia international went on to become the first foreign captain of the Andalusian outfit since Diego Maradona and led them to the first of their historic treble of UEFA Europa League titles in 2014 before he joined Barcelona.
Luis Suárez (Ajax to Liverpool, 2011)
The Reds wasted no time spending their windfall from Fernando Torres's sale to Chelsea, bringing in Andy Carroll and Suárez. Carroll managed 11 goals in 58 games before departing a year and a half later. Suárez fared rather better with 82 in 133 outings, including 31 as Liverpool fell just short of the 2013/14 Premier League title. It took Barcelona a reported €82.3m to prise him away.
Andrea Barzagli (Wolfsburg to Juventus, 2011)
Barzagli was not a complete unknown when Juve swooped – he had 25 caps to his name and helped Wolfsburg to the 2008/09 Bundesliga title – but his career had stalled. The Bianconeri weren't put off, drafting him into a defensive unit alongside Leonardo Bonucci, Giorgio Chiellini and Gianluigi Buffon. They have won seven successive titles.
Marcelo (Fluminense to Real Madrid, 2007)
"A pearl that half of Europe wanted," president Ramón Calderón said of Marcelo when he joined Real Madrid aged 18. Over a decade on and four Liga titles and four UEFA Champions League winners' medals later, that pearl has yet to tarnish. Another acquisition from South America that winter, Gonzalo Higuaín, also proved a bit of a catch.
Klaas-Jan Huntelaar (Heerenveen to Ajax, 2006)
This signing was a double coup for Ajax: the most promising striker in Dutch football and a player rivals PSV once had on their books to boot. Huntelaar scored 105 goals in 136 outings for the club before leaving, three winters later, for Real Madrid. He's now back in Amsterdam for a second spell.
Nemanja Vidić (Spartak Moskva to Manchester United, 2006)
Despite competition from Liverpool and Fiorentina, Sir Alex Ferguson signed the Serbian defender and his determined pursuit was vindicated in due course. There was no catching Chelsea in 2005/06, but Vidić's partnership with Rio Ferdinand provided the bedrock for an era of dominance, alongside another winter 2006 arrival, Patrice Evra.
Dejan Stanković (Lazio to Inter, 2004)
Financial problems meant Lazio could not refuse Inter's offer and it was quickly apparent the Nerazzurri had got a bargain. Stanković established himself as a firm fans' favourite by scoring from a corner against rivals AC Milan within weeks of arriving. He went on to win five Scudetti and the 2010 UEFA Champions League.
Jaap Stam (Willem II to PSV, 1996)
Not strictly 21st century, but well worth an entry. Stam was only at Willem II for six months but made such an impression that PSV swooped, seeking to end three seasons without silverware. The 23-year-old helped them to the Dutch Cup in his first campaign and the Eredivisie title in his second before joining Manchester United in 1998 for what was then a record fee for a defender.