No matter how many I download and try I can’t find any samples that work for me (other than some obvious ones like snares), so is there a guide to making sounds with free software (preferably open source)?
What do you mean when you say they don’t work for you? They physically won’t load? They don’t fit with the style of music you’re trying to make? etc.
What type of samples are you looking for? Have you considered checking out some VST instruments instead of using samples? There’s lots of great freeware synths and drum machines out there which you can take advantage of.
None of them sound good, I have been through most of the sites on the free sample thread here.
I am mostly looking for some good bass. I have used ZynAddSubFX and it is a nice softsynth but that still doesn’t have quite the control I am looking for. I thinking something along the line of a waveform editor that can also generate sounds (from a specific hertz, etc).
Could you be a bit more vauge please? 
I’m trying to make a bass thats good for gabber so it needs to be really low and distorted; so I was wondering if there was a sort of editor that I could input a hertz value (lets say 50hz) and then mess around with it in a waveform editor (make it square or saw-tooth, etc) that also includes stuff like distorts.
Do you want a bass sound (guitar, synth, etc) or a bass/kick drum? Since you said gabber I’ll just assume you want the typical distorted kick drum. I’d say that 9 times out of 10 you’re not going to get the exact sound you want directly from the synth/sample, you’re gonna have to sculpt it a bit using things like eq, filtering, fx, etc.
So, let’s start off with a simple kick drum. Here are a couple of nice drum synths you can try:
Drumatic 3 by E-Phonic
Drumatic VE by E-Phonic
µTonic by Sonic Charge is also very nice if you’re willing to pay.
Now, let’s assume that the kick drum isn’t quite as deep/bassy as you want it to be. We’re gonna have to tweak it a little bit with an EQ. The EQ-10 built into Renoise is perfectly fine for this task, just boost the 50Hz and 100Hz levels until you’ve got something that tickles your fancy. If you prefer a more graphical EQ approach, Electri-Q posihfopit edition is a nice one to try.
Now you want to give the kick some distortion to really get things moving. The distortion unit in Renoise is quite handy for this, but I personally prefer to use mda:overdrive for this kinda stuff, it’s got a smoother sound which I think works better on drums.
Once you’ve got a nice, distorted kick going on you can try adding a lowpass filter on top and then sweeping the cutoff frequency to do little buildups/breakdowns in your track, or just to finetune the level of distortion, etc. The Filter2 unit in Renoise is more than enough to handle this task, but there are tons of freeware filters out there too which work well.
If you’re still determined to get a sound which is hitting on a specific frequency, use a frequency analyser such as Voxengo SPAN. Just drop it onto your kick drum track (or the master track), open the GUI, then watch the frequency graph as you play your kick drum and adjust the pitch/fx/etc. SPAN displays an average of the frequencies so it is very easy to see certain peaks in the sound appearing on the graph. Simply tweak your sound until you see a nice peak appearing around the frequency you want, 50Hz for example. You can do this in Renoise too using the master spectrum view, but the graph reacts a lot faster in Renoise making it more difficult to work with for this task.
Hope some of this helps.
Thanks, that was really helpful.
Cool. Be sure to share the results if you make anything nice. 
How do you install VST’s? I searched this forum, but I couldn’t find the answer.
There is probably something in the manual.
Basicly just install the VSTi to a directory that suits you.
If the VST is just a compressed file you’ll find one or more .dll’s (assuming windows here).
Create a directory under the folder you want to put it (to make it more tidy).
Then in Renoise’s preferences you add the top directory in the VST settings.
Once Renoise scan that directory it will pick up the VSTs in it’s subfolders.
e.g.
C:\program files\vsts <- make that the target in renoise.
c:\program files\vsts\Vanguard <- create subfolders for the different instruments and effects.
Do what the last guy said…
Or alternatively you could RTFM 
Sometimes I used a free sound editor called Audacity to generate sine or square sounds through a menu it has. You only tell it the frequency of the square wave and the amount of seconds and it generates that waveform for you.
Then you can cut, paste, normalize, distort, etc. inside Audacity before loading the sound into Renoise.