Apps with 'Install Syslinux' feature

MultiBootUSB is a software to create multi boot live Linux on a removable media i.e USB disk. It is similar to UNetbootin but many distros can be installed, provided you have enough space on the disk.

WinSetupFromUSB is a Windows program, which prepares multiboot USB flash or fixed disk to install any Windows versions since 2000/XP, boot various Linux and *BSD flavors, as well as many Windows, Linux, DOS based and other utilities. Since version 1.

RMPrepUSB allows the Windows user to easily and quickly 'roll their own' multiboot USB drive.

SharpBoot is a small utility for creating multiboot ISO or USB. You just need to drag-n-drop some iso files inside, and it automatically adds description and category (for supported ISO files, not shown in the screenshot). It uses Syslinux for the menu and bootloader.