There are a lot of workbench accessories on the market, but typically they're either something that needs to be built as part of the workbench top and are nearly permanent components or they're small useful items, but don't have much versatility to them.

A newer accessory that hit the market in the past year is the Veritas Surface Vise. It's a great tool that adds a ton of versatility and options to my less than adequate bench. The basic tool right out of the box worked amazing, and because it's removable head is designed to have shop made accessories attached...if necessary...the options for it's use increases dramatically.