XcrysDen is a crystalline and molecular structure visualisation program, which aims at display of isosurfaces and contours, which can be superimposed on crystalline structures and interactively rotated and manipulated. It can run on most UNIX platforms, without any special hardware requirements. If you’d like to use XcrysDen to visualize VASP output, then you’ll probably want to check out v2xsf: a command line tool that converts structural output data generated by VASP (crystal structure, charge density, electron localization function, and local potential) to a format that can be read by XCrysDen.