Lesserton & Mor (L&M) is a complete site-based sandbox resource. It is essentially a detailed frontier city (Lesserton) and a nearby adventure site (Mor). The Mor book is a blank map and several random systems to help the DM determine the contents of Mor, a huge ruin inhabited by fearsome monsters and littered with treasure. The Lesserton book is a more traditional guide to a thriving fantasy city, complete with eccentric characters and plenty of opportunities to fritter away money and make new acquintances.
Conceptually it's bit like Pavis and the Big Rubble for RuneQuest, but more like dark ages Britain.
All in all, it's good stuff and one could easily run a long and involved campaign without ever leaving the environs detailed in L&M. Although set in the officially implied world of Labyrinth Lord, it is easily portable to just about any kind of traditional D&D campaign. It's very 'British' in tone and setting though so if that annoys you, some light reskinning may be in order. Check out the free Players Guide download; it will give you a good idea of what I mean.
Personally I think that L&M is a great sandbox resource for LL/OD&D and I look forward to running it. If you're looking for a prefab sandbox campaign, you could do much, much worse than L&M.