List of vendors and their associated driver support information gleaned from the net.

• Lucent / Agere WinModems. Unfortunately, this chipset/driver combination does not support multiple instances.
• PCTel chipsets have found their way into a large number of systems. The best source for drivers appears to be at the PCTel section of the Technion University website.
• Rockwell
• Conexant chipsets have Linux drivers built for them by Linuxant. Just as every other soft modem vendor out there, Conexant is not making the entire driver freely available and thus the purists will have to put up with the tainted kernel complaints. Acoording to the Linuxant INSTALL page for the HCF controller-less driver, multiple instances will work fine. However, seeing that nothing is free, this driver costs money to operate at speeds above 14.4Kbps!
• Motorola provides its own (tainted) drivers for the SM56 family of modems
• Cirrus Logic CL-MD5620DT
• Intel MD5628D-L-C and DQ82536EP chipsets
  Pre-compiled binary drivers for any number of Linux flavors can be downloaded from Intel, but please leave a message for them complaining about the lack of source code!

A good summary of the state of Linux support for WinModems is also available at Modemsite.com although you cannot download anything from them without paying a subscription fee.
A well-maintained, open site on all things LinModem is maintained by Jacques Goldberg at Technion University in Israel.