Dial-up

The most significant difference between modems for Linux isn't whether they're internal or external. They are whether they are a softmodem or a hardware modem (intermediate "controllerless" modems also exist). A Softmodem, or software modem, is a modem with minimal hardware capacities, designed to use a host computer's resources (mostly CPU power and RAM but sometimes even audio hardware) to perform most of the tasks performed by dedicated hardware in a traditional modem. All external serial modems are hardware modems. Some external USB modems may work in Linux.

Software Modems
They are also referred to as a Winmodem because the first commercially available softmodems mostly targeted the Microsoft Windows operating system running on IBM-PC compatibles. Although their usage has become more widespread over other operating systems and machines e.g. embedded systems and Linux, they are still difficult to use on operating systems besides Windows due to lack of vendor support and lack of a standard device interface.

If the box says the modem is "host-based", "HCF", "HSP", "HSF", "controllerless", "host-controlled", "soft modem" or "designed for Microsoft Windows" then it is a Softmodem.

Hardware Modems
A hardware (controller-based) modem will have three components: The Microcontroller Unit (MCU), the Data Pump Unit (DPU), and the Data Access Arrangement (DAA).

Chipsets

 * Rockwell RCV336ACF/SVD
 * Conexant SmartSCM
 * Intel MD565X/MD566X
 * US Robotics 5610b
 * Texas Instruments 405/504
 * Texas Instruments 415/514
 * Texas Instruments D1164X
 * Texas Instruments 700

Modems

 * 3com US Robotics 5610b (Amazon, Google Products)
 * 3com US Robotics 5610c (Amazon, Google Products)
 * US Robotics 802972A Performance Pro (ebuyer UK, Google Products UK)
 * Lectron Pragmatic I56LVP-F3
 * Multi-Tech Systems MultiModem PCI, Model MT5634ZPX-PCI (Amazon, Amazon UK ISA version)
 * Archtek Telecom Smartlink 5634PCV Surfrider
 * Digitan Systems Digitan Model DS560-558
 * Zoom 2920 (Amazon, Google Products)
 * L56DV 	Actiontec Model PCI56012-01
 * L56DV 	Actiontec Model PM560LKi
 * Gateway Telepath 6001022 "Deadwood"
 * All external serial modems

Software
Gkdial or wvdial.

Buying

 * Puppy Linux discussion
 * Ubuntu Forum Discussion
 * Slashdot article
 * Geektime Linux
 * LinuxQuestions List of compatible modems
 * X-Modem
 * LinModem

Setting up

 * Linux Forums 1
 * Tech Support Guy
 * Linux Questions 1
 * YoLinux tips for AOL
 * Linux Questions Wiki
 * Linux Mag