The Gemini missions of 1965-1966 were key to the success of Apollo. Mercury just did not do enough for all that the astronauts needed to make a Moon landing a success.

Indeed, it took almost all the missions to get EVAs right, thanks to Buzz Aldrin in Gemini 12. And docking two spacecraft was not easy, either - just ask Gemini 8, which included some guy named Neil Armstrong on his first space mission.

Gemini was also a fascinating era of manned space exploration. More technical and innovative than Mercury, but not as glitzy as Apollo. The astronauts who flew those missions said they liked Gemini because they got to use their piloting skills so often.

Yet another era of space that is gone. Unless the private efforts succeed, then we may yet again return to those pioneering days.

http://science.ksc.nasa.gov/history/gemini/gemini.html

http://www-pao.ksc.nasa.gov/kscpao/history/gemini/gemini.htm

http://www.thespaceplace.com/history/gemini2.html