Has there been any inexpensive item that enhanced your system's sound?

Test/Burn-In discs are the way to go!

Uh, sound panels. They're like $50 each. And bass traps too. Using sound panels is, without doubt, the best dollar-for-doaalar improvement you can possibly make. Just at the first reflection points (walls and ceiling) will make a huge difference.  

Dramatic changes playing with resonance control, on the motor pulley and especially on the tonearm (Trans-Fi air bearing). The Koetsu changes character from mellow to aggressive, depending on the application of small fragments of sticky jelly pad, which is sold to tune the sound of drums. Seems that suppressing odd harmonics is the key to clarity plus listenability, while suppressing even harmonics leads to shrill. Anyway, less than a gram required.

Moongel is the name of the product, price $10.

I have found that the quality of the power supply voltage significantly affects your sound quality.  With all the talk of power cords, outlets and everything here.  I would say the biggest inexpensive element I have found are Isoclean fuses.  These are the cheapest fuses ($49 retail) and they have improved things in every piece of equipment I have put them in.  I have tried other fuses, including Hi-Fi Tuning, SR 20, SR RED, Furutech.  Each of them had good points and bad points.  Each introduces a character to the sound.  Some of these may be good in your system depending on system synergy.  However, I feel the Isoclean made the best and most neutral improvement overall.  It provides a very natural and full sound and fixes a lot of brightness problems with stock fuses.  Other fuses may compensate for different problems.  The SR RED can improve a system that is too bright (it's actually the most non-intrusive fuse, but it reduces the resolution of the sound).  The SR 20 can improve a slow system, such as tube, but it's too bright and bass lean for most solid state.  Hi-Fi Tuning put a metallic edge on the sound that I didn't like. 

There are quite a number of very effective tweaks. Of these, I believe the Schumann resonance devices have made the most dramatic positive improvement.