SampleRadar, "the hub page for MusicRadar's regular giveaway of pro-quality, royalty-free samples", is one of the greatest sources of royalty-free samples out there. On the main page (linked above), they claim: "Because they're royalty-free, you're welcome to use them in your music in any way you like - all we ask is that you don't re-distribute them".
Free Sample Packs "provide exclusive free samples to use in your music royalty free", although, as claimed, "we try to ensure the free sample packs listed on this site are 100% royalty free – however there may be the rare occasion when this is not the case. Always read the description to find out if the free samples are royalty free or not, the responsibility lies with you and not us to ensure this is carried out". Any sample pack in this list will be checked by reading the comment under the pack.
Cymatics' "Ultimate List of Royalty Free Sample Libraries" have "put together a list of the best free sample libraries that are 100% royalty-free for you to use".
Black Lotus Audio has a "Free Downloads Vault", which comes packed with Serum presets, as well as drum, melody and vocal sample packs.
Exclusivemusicplus' sample pack collection features a lot of sample packs, plenty of which are labeled with "100% Royalty Free"!
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SampleRadar - 200 free drum break samples
Slower drum breaks, great for hip hop. One-shots not included. -
SampleRadar - 282 experimental breaks samples
Very odd, slower breakbeats, with phaser- and flanger-like effects applied on top. One-shots not included. -
SampleRadar - 286 free bespoke breaks samples
Some D'n'B and hip hop breakbeats. One-shots not included. -
SampleRadar - 355 free breakbeat samples
Slower drum breaks, great for hip hop. One-shots included for most. Also has bass, FX and synth loops. -
485 free DnB essentials samples
Great breakbeats for DnB, jungle and breakcore. One-shots included. Also has bass and pad loops, as well as some bass, chorus and FX one-shots. -
328 free drum 'n' bass samples
Great breakbeats for DnB, jungle and breakcore. One-shots included. Also has bass loops and one-shots. -
Hardcore Junglism's "Free Sample Pack Vol 1"
Breakbeat loops. Also some less notable melodic and bass loops, as well as bass stabs.
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Cymatics' "Orchid"
Hip hop and RnB drum one-shots and loops, as well as melodies. -
Cymatics's "Cobra"
Hip hop and trap drum one-shots, FXs, 808s and melodies.