Many companies manufacture MIDI sound module hardware to recreate dozens of instruments. Roland offers the Fantom-XR, making the famous sounds from their Fantom keyboards available for MIDI use. The portable Sonic Cell from Roland allows a guitar to be used as a controller, making MIDI available to guitarists unskilled at the keyboard. Muse's Receptor 2+ stands highly regarded by professionals for its processing speed and sound quality. Yamaha's MOTIF RACK-XS offers real-time sound manipulation, and a multi-mode for full sound production.
Native Instruments produces two high quality MIDI software sound modules in their Komplete series and their groove-based Maschine products. Steinberg's powerful Halion software allows for integration with other manufacturer's sound libraries, and offers 160 different instruments in its own library. Spectrasonics Omnisphere packs over 8,000 sounds in its sonic library, providing immense versatility; and includes the world's first Arpeggiator able to match the groove of Stylus RMX and MIDI files.
Specialty modules often come in the form of a 19" rack mounted device, but keyboard and micro modules exist as well. This group specializes in recreating specific sounds. Waldorf and Access' Virus line stand at the top of this class. Hammond makes equally outstanding products in line with their historic organs.
MIDI sound modules don't have to cost a fortune. Freeware such as Illformed Glitch, Green Oak Crystal and MDA ePiano supply excellent sound and creative options at the right price. Many amateur programmers make their products available for free on the Internet. Quality and stability vary, so do some research before clicking the download button.