I use MongoDB as my database of choice as you can tell by my blog entries. Under current Raspberry Pi OSs, MongoDB is limited by 32 bit binaries. Databases are restricted to 2GB.
The Raspberry Pi 3 sports an ARMv7 processor that supports 64 bits. At this time (May 2016) there is no official OS support for 64 bits. Raspbian and Arch Linux are only available with 32 bit support. I expect that we WILL see 64 bit support sometime in the future 😉
While waiting, I looked around and found MongoDB
3.2.6 3.2.10 in the Arch Linux ARM aarch64 package repository. I also found out that the ODROID C2 single board computer supports aarch64. This SBC costs $40 and has better specs (for my DB purposes) than the R-Pi 3. It has a 4-core ARMv8 processor running at 2GHz, 2GB of RAM and gigabit ethernet. It also supports WAY fast eMMC Flash storage in addition to Micro SD. Android and Ubuntu are the officially supported OSs but Arch Linux ARM (64 bit) can be installed as well.
[UPDATE: 23 Mar 2017] If you prefer to use the default Ubuntu Linux for ODROID C2 as opposed to Arch, you can now install official MongoDB 3.4. See this blog post.