GLint num = 0;
glGetIntegerv(GL_NUM_EXTENSIONS, &num);
std::cout << num << std::endl;
while(0<--num) {
    std::cout << glGetStringi(GL_EXTENSIONS, num-1) << std::endl;
}

// Output:

20
GL_APPLE_texture_range
GL_APPLE_row_bytes
GL_APPLE_rgb_422
GL_APPLE_object_purgeable
GL_APPLE_container_object_shareable
GL_APPLE_client_storage
GL_EXT_texture_sRGB_decode
GL_EXT_texture_mirror_clamp
GL_EXT_texture_filter_anisotropic
GL_EXT_texture_compression_s3tc
GL_EXT_framebuffer_multisample_blit_scaled
GL_EXT_depth_bounds_test
GL_ARB_timer_query
GL_ARB_texture_swizzle
GL_ARB_shader_bit_encoding
GL_ARB_separate_shader_objects
GL_ARB_occlusion_query2
GL_ARB_instanced_arrays
GL_ARB_explicit_attrib_location