To confirm: "IT mode" is an operating mode in which each disk controlled by a HBA is visible to the operating system. By contrast, a HBA operating in a RAID mode ("IR mode") abstracts multiple disks the HBA controls as one disk that the operating system sees. FreeNAS and other software-based RAID systems should be run with HBAs in IT mode because those software-based systems want disk-level access and control.
The HBA decision should be made as part of a larger decision about what you're trying to achieve. Many people fall into one of two camps, either a smaller but faster storage installation or a larger but slower storage installation. Naturally, we'd all like a larger, faster installation but that gets expensive for most people. Your hard disk storage strategy will drive your HBA decision (at least to the HBA generation) because the largest component of your cost - and the largest cost variability between strategies - will likely come from your disk selection strategy. The discussion that follows assumes an 8-port HBA although 4-port or 16-port cards are available (but less common).
A smaller/faster installation might be up to 8 smaller SAS3 disks running a 93xx-generation card like the M1215 as
@i386 said. If you use a Supermicro case in conjunction with this installation, you would not need an expander backplane for your 8 ports (which may be cost prohibitive for SAS3) and you could use a TQ backplane instead (i.e., direct connect). SAS3 disks are several times more expensive than SAS2/SATA3 disks for the same capacity, however.
A larger/slower installation might be up to 24 SAS2/SATA3 disks (such as the 3TB HGST disks recently discussed in the deals forum) on a SC846 box with a SAS2 expander backplane. A M1015 HBA would be fine for that if flashed into IT mode. Alternatively, a 9207-8i card already comes in IT mode. This configuration is common and inexpensively available.
Regardless of your HBA choice, you'll want to ensure that the HBA uses the latest firmware - currently P20. Flashing or updating firmware is not hard and you'll find good guides that are beginner-accessible to be readily available.
So, depending on your overall goal (large/slow or fast/small), your HBA choice should follow from that goal.
