function guiSetup()
guiButton = function(x, y, c, t)
w = tpt.textwidth(t)
h = 7
local c = c
if not c[4] then c[4] = 255 end
local k = 3
if guiMousebound(x - 2, y - 2, x + w + 3, y + h + 3) then
k = 1.5
end
tpt.drawrect(x - 2, y - 2, w + 3, h + 3, c[1], c[2], c[3], c[4])
tpt.fillrect(x - 2, y - 2, w + 3, h + 3, c[1], c[2], c[3], c[4] / k)
tpt.drawtext(x, y, t, c[1], c[2], c[3], c[4])
return x - 2, y - 2, x + w + 3, y + h + 3
end
local guiclicked = function(x, y, b, e)
if e == 1 then
guiClick = 'press'
elseif e == 3 then
guiClick = 'hold'
elseif e == 2 then
guiClick = 'release'
else
guiClick = nil
end
return false
end
tpt.register_mouseclick(guiclicked)
guiMousebound = function(x1, y1, x2, y2)
if type(x1) ~= 'number' or type(x2) ~= 'number' or type(y1) ~= 'number' or type(y2) ~= 'number' then return false end
return tpt.mousex >= x1 and tpt.mousex <= x2 and tpt.mousey >= y1 and tpt.mousey <= y2
end
guiWindow = function(x, y, w, h, c)
tpt.drawrect(x - 1, y - 1, w + 2, h + 2, c[1], c[2], c[3], c[4])
tpt.fillrect(x, y, w, h, c[1], c[2], c[3], c[4] / 5)
end
end
guiInit = function()
guiSetup()
if guiDraw then
tpt.register_step(guiDraw)
end
end
guiQuit = function()
if guiDraw then
tpt.unregister_step(guiDraw)
end
end

...And some example code:

function guiDraw()
if not b1 then
b1, b2, b3 = 0, 255, 0
end
guiWindow(100, 100, 400, 180, {b1, b2, b3, 255})
local str, greetA
if tpt.get_name() == '' then
str = 'Hello! If you sign in, then I may be able to greet you properly!'
else
str = 'Hello, ' .. tpt.get_name() .. '!'
end
if guiMousebound(102, 89, 102 + tpt.textwidth(str), 96) then
greetA = 255
else
greetA = 125
end
tpt.drawtext(102, 89, str, 255, 125, 0, greetA)
a, b, c, d = guiButton(104, 104, {255, 255, 255, 255}, 'I tell you when I\'m pressed!')
be, bf, bg, bh = guiButton(104, 117, {255, 125, 0, 255}, 'Random Background')
i, j, k, l = guiButton(104, 270, {255, 0, 0, 255}, 'Quit')
if guiMousebound(a, b, c, d) and guiClick == 'hold' then
tpt.drawtext(c + 2, b + 2, 'Button was pressed!', 255, 0, 0, 255)
b1, b2, b3 = 0, 255, 0
elseif guiMousebound(be, bf, bg, bh) and guiClick == 'hold' then
b1, b2, b3 = math.random(0, 255), math.random(0, 255), math.random(0, 255)
elseif guiMousebound(i, j, k, l) and guiClick == 'hold' then
guiQuit()
else
b1, b2, b3 = 0, 255, 0
end
end
function guitest()
guiInit()
end

To use the sample, type in the console 'guitest()'. It looks really simple on the outside, but it's pretty complex on the inside. 

If you come up with any really good scripts using this, post them here!