Why does my resistor get warm?
This can be easily calculated.

The power supply is 5 V and the LED drops 2 V.  That leaves 3 V accross the resistor.  3 V / 150 &Omega; = 20 mA, which is a typical max current for small LEDs.  That means the LED is driven correctly.

Now look at the power dissipation.  20 mA x 3 V = 60 mW.  That's well within the capability of what looks to be a "1/4 W" resistor in your picture.  Again, everything is fine.

Dig out a datasheet for the resistor and see how hot it is expected to get if you actually were to have it dissipate 1/4 W.  That would probably be in the 150-200&deg;C range.  Even at 150&deg;C for 250 mW, and assuming 20&deg;C for ambient, you have 130&deg;C / 250 mW = 520&deg;C / W.  60 mW would therefore heat the resistor 31&deg;C, which you can definitely feel.  If starting at 20&deg;C, then the resistor would be at 51&deg;C, or 124&deg;F.  So it makes perfect sense that it would feel "warm" or almost "hot" to you.