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

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.

Regarding power isolation, buy products that solve the solution inherently. Insted of chasing it after the fact.  The line currently offered by Vinnie Rossi are an amazing all in one solution. I have a VR120 that is sublime. Lots of power/current and puts out such an organic relaxed sound. 

Four Herbie's Audio Lab "Big Fat Dots" under each of my KEF LS50 monitor speakers that sit on home made concrete block speaker stands.

There's always wrapping the transformer with mu metal.  Low frequency high permeability mu metal.