Thecus' N2350 is a two-bay SOHO NAS with decent performance that won't empty your wallet.
Synology's DS1817 provides high capacity and 10 GbE connectivity at a reasonable price.
QNAP's TS-453B is an expensive four-bay NAS, even before adding optional 10GbE.
Updated - Synology's DS1517+ is an attractive choice for a 10GbE capable NAS.
QNAP's TS-431X is an inexpensive way to move up to a 10 GbE NAS.
QNAP's TS-231P is a good value in a RAID 1 NAS.
QNAP's TS-228 is inexpensive, but makes performance compromises to get there.
QNAP's TS-431P is an entry-level RAID 5 NAS that should satisfy cost-focused buyers.
Synology's DS416 and DS416j are both good values in four-bay RAID 5 NASes.