The only spacecraft to ever visit Neptune is the
Voyager 2 probe. The Voyager 2 probe was
launched on August 20, 1977 and reached Neptune on
August 24, 1989. The Voyager 2 probe detected faint
bands as well as some faint cloud patterns on
Neptune.
In addition, you may want to check out
AeroSpaceGuide. This site has some interesting
facts as well as mission summaries.
There are no further plans to send a
probe to Neptune.