VirusTotal is a great analysis platform for enriching data on IOCs and finding related malicious infrastructure. VT inspects IOCs with over 70 antivirus scanners and URL/domain blacklisting services. The platform offers a search engine for previously scanned items, as well as a number of URL and file submission methods, including the primary public web interface, desktop uploaders, browser extensions and a programmatic API.
In our opinion, VT's holy grail is its awesome VT Graph - a dynamic threat relations visualization, allowing users to view information about each entity, pivot over data points, edit the graph, and add new nodes. Users can also save their graphs and download the node list.