Here's what I'm running, it's basically a generic Celeron N3150 box(you're basically free to pick whatever you want), no moving parts beyond the on/off switch - totally silent -
ASUS N3150IC motherboardDell Intel Port Dual Port Gigabit PCI-E Network Card X3959 - used, from eBay
MI 008 case - cheapest case I could locate, you may have other preferences.
PicoPSUSSD
Only thing you have to be careful with in the case of pfSense is making sure your card works(not just Realtek vs Intel but things like quad port vs dual port IIRC). Apparently Intel cards just work much better i.e. you won't necessarily see a difference in throughput between Realtek and Intel but underneath the hood people have posted that Realtek puts more strain on the system(many more interrupts and CPU load).
In regard to the case it was the cheapest I could find, but at the time I couldn't find it in the UK, so I purchased it on Amazon France (£31 delivered - I shall now await the smart alec reply from somebody who does actually know where to buy one in the UK cheaply
).
I've just upgraded pfSense to 2.3 and the new UI is looking very slick and modern ... I've been through a lot of routers Cisco, Juniper, ASUS, Thomson, etc, etc. Basically I just got fed up with proprietary stuff and decided that cheap, powerful, commodity hardware, and free software was the way to go.