AST FourPort and clones (4 port)
Accent Async-4 (4 port)
Arnet Multiport-8 (8 port)
Bell Technologies HUB6 (6 port)
Boca BB-1004, 1008 (4, 8 port) - no DTR, DSR, and CD
Boca BB-2016 (16 port)
Boca IO/AT66 (6 port)
Boca IO 2by4 (4 serial / 2 parallel, uses 5 IRQ's)
Computone ValuePort (4, 6, 8 port) (AST FourPort compatible)
DigiBoard PC/X, PC/Xem, PCI/Xem, EISA/Xem, PCI/Xr (4, 8, 16 port)
Comtrol Hostess 550 (4, 8 port)
PC-COMM 4-port (4 port)
SIIG I/O Expander 4S (4 port, uses 4 IRQ's)
STB 4-COM (4 port)
Twincom ACI/550
Usenet Serial Board II (4 port)
Non-intelligent cards usually come in two varieties, one using standard com port addresses and use 4 IRQ's, and another that's AST FourPort compatible and uses a selectable block of addresses and a single IRQ. (Addresses and IRQ's are set using setserial.) If you're getting one of these cards, be sure to check which standard it conforms to, prices are no indication.
Computone IntelliPort II (4/8/16 port)
Cyclades Cyclom-Y (RISC-based, 8-32 ports) (ISA/PCI)
Cyclades-Z (high-end, 16-64 ports) (PCI)
DigiBoard PC/Xe (ISA), PC/Xi (EISA) and PC/Xeve
Equinox SST Intelligent serial I/O cards
Hayes ESP 1, 2 and 8 port versions Included in kernel since 2.1.15. The driver for kernel versions 2.0.x can be found at (driver)
Stallion EasyIO (ISA) / EasyConnection 8/32 (ISA/MCA) / EasyConnection 8/64 (PCI) For DIP switch settings and configuration files check (driver)
Stallion EasyConnection 8/64 (ISA/EISA) / ONboard (ISA/EISA/MCA) / Brumby (ISA) The latest driver can be found at (driver)
Comtrol RocketPort (8/16/32 port)
(driver) (kernels 1.2.x). A driver for kernels 2.x can be found at (driver)
DigiBoard COM/Xi Contact Simon Park (si@wimpol.demon.co.uk) or Mark Hatle (fray@krypton.mankato.msus.edu). NOTE: both email addresses seem not to exist any longer.
Moxa C102, C104, C168, C218 (8 port), C320 (8/16/24/32 expandable) and C320T
RISCom/8
Specialix SIO/XIO (modular, 4 to 32 ports)
Specialix IO8+ Contact