I've been using the Paulstretch feature in Audacity for a lot of my pads and atmospheres lately. You can time-stretch audio by ludicrous amounts and create really interesting textures. Like take a 3 second synth or vocal sample and use the x40 time-stretch and you get a few minutes worth of crazy space sounds.
Someone even put the Windows 95 startup sound through it and it sounds amazing lol
The 'Valhalla Super Massive' plugin is worth getting as well. It's a free multi-fx (Reverb/Delay) plugin that can create some proper out-there sounds when you start pushing the settings up high:
Make some space. ValhallaSupermassive has been designed from the ground up for MASSIVE delays and reverbs. Get ready for luscious clouds of reverb, otherworldly delays, and swelling waves of feedback unlike any you’ve heard before. Supermassive has 21 out-of-this-world reverb/delay modes...
valhalladsp.com