The HubSpot free CRM is free and intuitive. They offer a startup program for discounted rates, if your needs marketing needs exceed their free tier.hubspot.com
Salesforce specializes in customer relationship management. It can easily scale to enterprise level, but is more expensive than other options.
Affinity is a relationship management platform that helps you leverage relationship intelligence to expand your network, support your portfolio, and increase proprietary deal flow.
Airtable is an easy-to-use online platform for creating and sharing relational databases. The user interface is simple, colorful, friendly, and allows anyone to spin up a database in minutes.