Personally I only really use the NEBuilder tool for designing primers for Gibson Assembly but I know other people in my lab use the double-digest finder for restriction cloning.
Primer BLAST is my favorite tool for primer design since it checks for primer specificity against a given genome:
https://www.ncbi.nlm.nih.gov/tools/primer-blast/
addgene.org isn't really a tool, but it's my go-to when I need to find plasmid sequences since they also carry maps/sequences even for plasmids they don't have in their physical repository.
Eugen's answer just reminded me of another one: http://www.molbiotools.com/restrictionanalyzer.html
Does restriction digest analysis and shows a mock gel of all the predicted products, I've even used it on whole bacterial genomes to figure out the best enzyme to give me a good range of product sizes for Southern blotting. It also has options for dam/dcm methylation in case the enzyme might be methylation sensitive when the DNA is extracted in some E. coli strains.