Available for download, AmpliTube 4 from IK Multimedia provides amplifier, cabinet, stompbox, and effects-rack emulations for electric guitars and electric basses. In stand-alone mode, this software can also function as an 8-channel DAW with full recording, looping, editing, mixing, and exporting capabilities. Used in any application such as Pro Tools or Logic, the software allows you to process DI'd guitar and bass through simulations of various guitar-related gear-pieces, and thus approximate a more realistic tone.
Essentially, AmbliTube 4 provides you with an emulated pedalboard, a simulated effects loop, a modeled amplifier, a cabinet stage, and an effects rack, all of which are yours to tweak however you see fit. Not only that—you get two sets of the aforementioned items, and you're able to route your signal through each model in any manner you choose; mono chains, stereo setups, and dual stages of amplification are possible, as are creative combinations in practically any assortment you could cobble together.
It is also worth noting that the tools in AmpliTube 4 work well in conjunction with analog synths, electric pianos, drums, and even soft-synths. This software is compatible with Mac and Windows platforms, and will work in VST2, VST3, AU, and AAX formats running at 64-bit.