Great question. Anything that furthers the field and does not risk becoming irrelevant in 6 months would be an excellent bet. + Utilities for computers and di

Great question. Anything that furthers the field and does not risk becoming irrelevant in 6 months would be an excellent bet. + Utilities for computers and disks are always useful and don't need much maintenance for long-term function/feasibility. + web frameworks that make web designing/dev easier (clojure[script], rails, python+django are all popular choices) + Look for a tool you commonly use or a service that's commonly requested.