Highrise is basic, web-based CRM software that works great in coordination with Basecamp. It's main features include sharing contacts, assigning tasks, keeping history and making cases.
A great feature is that you can add information, tasks and contacts to Highrise via email. Another great feature is assigning permissions for access. You can assign permission to everyone, only yourself, specific groups or specific people using dropdown menus.
Pricing starts at free for one Highrise user and increases in tiers depending on how many users and features you want.