Chris Porter's Blades contract has been terminated to enable the striker to join Colchester United.
The 31-year-old joins the Essex side after three-and-a-half years with the Blades.
Porter scored 23 times for the Blades in 118 appearances, 49 of them as a substitute. He has scored a number of important goals, including two against Nottingham Forest late on in last season's FA Cup tie. Ironically, he scored twice at the Weston Homes Community Stadium last season.
"We wish Chris all the best with the next chapter of his career. He played a substantial role in our FA Cup exploits last year but it has not worked out this season. We have worked with Chris at Derby and Sheffield United and he leaves on good terms," said Blades chief Nigel Clough.