Disclaimer: I am only slightly above noob status when it comes to recording. It's been a hobby of mine for a few years but I don't have a very broad sample of gear to compare from. These are just a few of the pieces that I've found to be reliable and worth their cost.
ART Pro MPA II - It's not hard to find these for a bit over $200 and it's a great way of adding a pair of extra preamps to a recording setup. I'm happy with mine and the general consensus seems to be positive.
Avantone CK-6 - At about $180, this is a solid microphone choice. It's gorgeous, well built, and sounds good. Paired with the MXL 4000 in mid-side configuration, I'm laying down some great (for my standards) solo acoustic tracks. I haven't encountered a lot of discussion on this mic so, for all I know, there are better options for the money but I'm in no rush to ditch the Avantone.
Focusrite Scarlett 18i6 - I've gone through a handful of low-budget interfaces (Presonus Audiobox, Presonus Inspire GT, TASCAM US1800, TASCAM US800) and the Scarlett has been, without a doubt, the best of the pack. I am tempted to recommend the Scarlett 2i2 but, by comparison, the 18i6 just offers a significantly better value. The expansion opportunities are unlike anything I've come across in a sub $300 interface, and in a small package. As an added bonus, they include the Scarlett Plugin Suite that has one of my favorite compressors. Pound for pound, the Scarlett 18i6 gets my vote for best bargain in the interface category.
ART Tube MP - Heck, it's $30. There aren't a lot of options for adding cheap channels to your rig at this price point. Even when I added the Pro MPA II to my rack, the Tube MP still finds a way to be useful. If you're desperate, give it a shot. There's not a lot to lose and you may be surprised at what it is capable of. I like mine quite a bit.
GLS ES-57 - The original post mentions that the SM57 is an obvious choice (and I agree) but, in terms of value and low cost, this copy is a winner. I wasn't expecting a lot from a $30 mic but it does a great job on guitar cabinets. Another worth mentioning is the Audix i5, which costs the same as an SM57. There's almost always a place for this breed of mic, regardless of whether it's the genuine Shure or the ultra-affordable GLS knockoff.
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My next $500 will probably be going toward some preliminary room treatment and a pair of competent monitors but the temptation to grab a compressor or a new preamp is fierce. It's probably best if I stay out of this thread for a while.