Draw Line

Is it possible to display a simple straight line between two objects?

Yes, you can either use a thin triangle, or you can use OpenGL's line drawing functions (look up GL_LINE). There's no in-built support for it at the moment.

Not with just 1 command.. you could get the viewport x/y for each object and draw a line between those 2 points, though that could pose problems if one of the entitys isn't visable. Or you could go a more 3d route and make an object (like a cylinder, cube or a sprite might be better for speed) and point it from one object to another (using PointEntity) and stretch it along it's z axis the appropriate length (use EntityDistance to get the length). a sprite would require a little extra tweaking so try it first with a cube.

Haven't tried either of those myself but it's an interesting effect idea... using linepick to find what something is pointing at you could make a nifty laser effect with the same idea...