MongoDB Atlas is the official cloud offering by the creators of MongoDB. And they are providing MongoDB in AWS, Azure and GCP.
They do offer a free tier which you can use for prototyping your apps on MongoDB Atlas. The free tier comes with following limitations.
1. No Region Choice
You can create your cluster only in N. Virginia AWS Region.
2. Shared RAM
You do get a Shared RAM.
3. 512MB fixed storage
Your Database Logical Size should not cross 512MB.
For a typical application this should be sufficient.
4. Bandwidth 1 GB in + 1 GB out per week
There a limit on total bandwidth you use. Its limited to 1GB in and 1GB out per week. If it crosses that limit then your bandwidth get throttles to 1KB/s
5. 100 databases and 500 collections only
You can at most create 500 collections only across 100 databases.