A Discone is a wideband antenna that is a great starting antenna for general RTL-SDR use. Over on instructables.com, user cyfus has created an instructable showing how to build a 55 MHz+ home made discone antenna for his RTL-SDR dongle. His instructions show how to build it out of parts and tools sourced entirely from hardware and electronics stores.
Using this antenna cyfus was able to receive signals from 25 MHz to around 900 MHz.
Reddit user soooooil has posted a series of images showing his home made Discone antenna made out of wire coat hangers and designed for a minimum frequency of 130MHz. His thread discussing the Discone is here.
A Discone is a good general antenna for the rtl-sdr as they are extremely wideband, and can be expected to be useful over almost the entire frequency range that is tuneable by the the rtl-sdr.
Another page on a home made Discone also designed for a minimum of 130MHz is here with some brief instructions links to design calculators.