Gibthon Construct Designer
is a software tool under active development to aid with the design of primers for Gibson Assembly. It currently comes in two flavours - Gibthon Beta, and Gibthon Construct Designer Beta.
Gibthon Beta - here
The more basic of the two tools, this will take as an input the sequences surrounding the join for which you wish to design primers. You can interactively adjust the length of the primers to tune the melting temperature of the DNA and check for secondary structures.
Gibthon Construct Designer Beta - Here
Gibthon's older brother, and much more powerful. With this, you can design your construct from its component genes and automatically generate the optimal primers. The tool has access to the Parts Registry Database, the NCBI's Nucleotide database, as well as accepting genbank formatted files from the user.