I have mixed and mastered peoples tunes yes!
 I'm actually going to start a little service where I offer to mix and masters peoples tracks but focus on trying to push getting mixes done and work with the artist to get mixes done before mastering because I'm sure as we know mixing is far more important than mastering but as stated above....some mastering engineers since it is a competitive and not necessarily lucrative field they just master the tracks and get the money! And no matter how good someone is at mastering, if the mixdown is shit, then the master is also going to be shit....just louder. Like a loud Shit.

There are many issues when working for other....first off ....mixing is (technically) subjective. How people hear and what people want to be prominent in a mix differs from each individual and each genre of music. Its hard to retain dynamics yet obtain that level of loudness for commercial release.......and this differs dependent on style. For example: some deeper progressive house focuses on dynamics, depth & soundscapes to try capture that atmosphere but something like dubstep is just slammed up against your limiter, squashed to shit and a big wall of sound....but that may be the sound a client is looking for so it is important to discuss this with the people whos music you are working on. Try to gain a sense of what they want but add your own interpretation.
 
Another issue is what you actually get given to work with. For example I prefer working with stems, since you don't want to drastically alter the artists creative intentions, ie....changing the reverbs on leads, altering how buildups or drops occur etc, however; sometimes with stems it poses limitations to the mixing / mastering. Here is one example: You could be supplied with a 'drum stem' where all the drum sounds are exported together in one WAV audio file....now you as a mixer may feel that the kick and snare relationship isn't sitting to well, simply in volumes, so it becomes a bit of a pain in the arse to alter simple stuff. Another example is if they have exported a 'BASS STEM' which houses a couple of bass sounds but their levels are way off, ie...subs way to loud in the mix......bit of a pain. I suppose the point here is to offer a service where you mixdown more stems. Tell them exactly what you want, eg: 'kick stem', 'snare stem', 'hats and percs stem', 'sub bass stem', 'main bass stem' etc.  The more things are separated from the beginning then the easier it is to approach.

A lot of people want you to mix direct from their project. If you have all the relevant plugins then it is great, however; 95% of the time....you both do not share the same versions of the DAW's or the same plugins so it just doesn't work.

Honestly, more often than not, producers have simply got their volumes of each track way off.......I tend to find people leave really sharp high frequencies in there or pump the bass way to loud to the kick....yunno simple things. Steve Duda once told a class something like 'I bet I can do a better mixdown than most people simply by adjusting volumes and not even processing stuff' .......I paraphrased there of course, but it was something along those lines. Steve wasn't being an arsehole he was merely pointing out that 90% of a mixdown is simply balancing volumes of your tracks and making space for each elemet. You gotta compensate the space and decide what elements belong in the foreground and what elements belong in the background. An artist doesn't create a one dimensional picture....the background is just as important as the foreground. The less things are fighting for space, the cleaner the mix is. Take current 'deep house'.....super clean mixdowns because there are so few elements.....drums and bass in foreground, some lead / piano / pad sat back a little....done!

so pay attention to volumes and what important elements belong to the front, which elements belong in the back, keep an eye on headroom for mastering, pay attention to phase,......even go as far as suggesting things that really aren't working in the track (although people do get shitty sometimes if you try offer constructive advice). As I said before don't try make them change their art but....gently suggest minor changes they could make to enhance the track.

when working on things for other people, the major issue is the complaints you get: ' I want it to sound like this', 'I want that', 'so n sos track hits at this rms and this doesnt' blah blah blah .....unfortunately some people just don't know what they want.....that's when you get mixing engineers or mastering engineers who stop giving a fuck and just do whatever haha!! Unfortuantlely the actual 'sound' of current popular dance music is actually a big fuck you to mixing engineers. Gotta try find that happy medium for most people.

also a big fan of the Valhalla frequency echo and also the Fab Filter timeless plugin. Get some really cool delay FX on that one.
Also one that I'm pretty sure is free is the TAL dubdelay ....really simple but can get some really cool dub delay FX. And of course if I want any simple delays my go to is the Bionic Delay, which I'm pretty certain is also free.